Abstract: Abstract: Machine-transplanted rice had many advantages, such as maintaining high efficiency, saving cost, freeing labor, and stabilizing yield, so the area of machine-transplanted rice becomes larger and larger in China in recent years. With the expansion area of machine-transplanted rice, the demand for high quality of rice seedling becomes more and more important. However, traditional seedling raising method such as using paddy soil as the seedbed substrate is difficult to meet the growing demand of machine-transplanted rice seedling. It is predicted that more new seedbed substrates for rice seedlings such as mixed substrate and light soilless substrate will replace nutrient soil for raising rice seedling. Organic waste substituted as raising substrate, such as straw seedling substrate is widely used on vegetable production. In recent years, we have developed an organic compound decomposing agent, which can decompose the straw rapidly. And also, straw composting plus straw decomposition agent can improve the quality of compost. Inspired by these, we need to study the effect of straw decomposition agent applied to straw substrate seedbed and machine-transplanted rice production. Field experiments were conducted with hybrid rice cultivar Hyou518 at the experimental farm of Jiangxi Agricultural University, Jiangxi Province, China in late-growing season in 2015. And the experiments were conducted under the condition of mechanical seedling and transplanting with high-yield agronomy technology during the whole growth period of late rice, and with three replicates for each treatment. In the field experiments, three types of rice seedbed substrates: straw substrate plus straw decomposition agent (T1), straw substrate without straw decomposition agent (T2) and paddy soil (CK) were used to compare their physical and chemical properties. The effects of different seedbed substrates on seedling and transplanting quality and yield formation of machine-transplanted rice were also analyzed. The results showed that, bulk density of T1 and T2 were 58.65% and 63.46% lower (P<0.05) than CK, while the porosity (aeration porosity and water-holding porosity) and water content were significantly higher (P<0.05) than CK (except water-holding porosity of T2). Inoculating straw decomposition agent to straw seedbed substrate was beneficial to improve the physical and chemical properties of straw seedbed substrate, reduce organic content, improve available nutrient concentrations, and decrease C/N ratio. Compared with T2 and CK, T1 had better seedling and transplanting quality, earlier and faster growth of tillering at early growth stage, and higher LAI, dry matter production and nutrient (N, P, K) absorption. Moreover, T1 had more panicles and total spikelet, and higher grain yield. The grain yield of T1 was 2.25% and 4.37% higher than that of T2 and CK, respectively. The results suggest that straw seedling substrate plus straw decomposition agent fulfilled the condition of seedling growth, and the seedlings were more suitable for current mechanized transplanting technology than paddy soil. In summary, the straw seedbed substrate added with the decomposition agent is a suitable seedling substrate of machine-transplanted rice because of its superiority on improving the grain yield and the utilization of straw resources. The production process technology of multifunctional substrate that produces strong seedling, improves soil fertility and disease resistance, and produces high yield still needs to be further studied, even though straw seedbed substrate from this study is more suitable for current mechanized transplanting technology than traditional seedling raising pattern. In addition, it is noted that this study just included one year experimental data in late rice field. Future studies on the validation and perfection of multiple years and sites are needed in the future.