Abstract: In order to better understand the microclimate inside a plastic covered multi-span greenhouse, it is essential to investigate the ventilation rate for the performances of various vents in it. For determining and modeling the ventilation rate, the experimental investigation was conducted by making use of the tracer gas technique and decay rate method in the greenhouses with continuous roof vent or with rolling-up roof vent. It was found that the rolling-up vent had a higher ventilation rate (up to 2.32 times) than the continuous roof one, the better ventilation (more than 60% at the wind speed of 3.0 m/s) could be achieved with vents near the ridge of the roof instead of near the gutter, and the obvious reduction (18%-22% at the wind speed of 3.0 m/s) in the rate values when an insect-proof screen was fitted to it, in the case of full opening of the vents. And the experiments confirmed the significant differences in the rate values when they were opened in the different known spaces by comparison between the two vents. Furthermore, for the simplification of the study, the wind effect coefficient Cw and discharge coefficient Cd were introduced and determined in values for this kind of greenhouse. The C, in relation to Cw and Cd, was obtained with the consultable value of 0.178 for the continuous roof vent, 0.318 for the rolling-up roof vent for the rates due to wind forces, and Cd was obtained with the consultable value of 0.667 for the continuous roof vent, 0.863 for the rolling-up roof vent for the rates due to buoyancy effect in related situations.