Rice blast is one of the main rice diseases. In order to discuss the occurrence regularity and improve the veracity of forecasting, the effects of ecological factors on infecting process, such as rice growth period, cultivar resistance, temperature, precipitation, and fertilization of nitrogen, were studied quantitatively. The results showed that adhering conidia on leaves, or ratio of infection was negatively correlative with the growth period of rice. The latent period of Pyricularia grisea was closely related to temperature, and within the range from 10 degrees C to 33 degrees C, the latent period was the shortest at 28 degrees C. The ratio of conidia adhering on leaves was also closely related to precipitation, especially the intensity and duration of precipitation in the first 5 hours after conidia contacting leaves, and there was no obvious effect after 5 hours. Under certain conditions, the ratio of expansive lesions to non-expansive lesions, the maximum area of expansive lesions, and variety resistance of rice were related to the rank of leaves.