Abstract: The characteristics of exhaust emissions with different mixing ratios of bio-diesel was studied. The pollutant formation mechanism of bio-diesel was discussed. The pollutant regulation of different mixing ratios of bio-diesel varying with engine rotation speed and load were also discussed. The optimal ratio of mixing bio-diesel was obtained from the study of engine emission under different working conditions. The experimental results show that the bio-diesel fuel can reduce its emissions effectively especially can decrease the particulate matter(PM). When the bio-diesel ratio increases, the emissions will be decreased at different degrees. Retarding fuel injection timing can also reduce NOx and PM emission effectively for bio-diesel engine.