Based on data of hybrid rice sown at different times, discussionis made of the increase in the weight of dry substance in conjunction with the behaviour of the Logistic curve, and of the b-value as anindication of the length of the growth period. The notions growthspeed (GS) and relative growth speed (RGS) are introduced and it isshown that when t=-a/b, GS attains its maximum and the RGS curveshown an inflection point, suggesting that substantial change take splace around the boot stage in the growth of the rice. In addition,the variation of optimum temperature for the growth of hybrid rice andthe effect of sunlight-temperature condition on the RGS are investi-gated and corresponding relationship is expressed. Finally a sunlight-temperature model is established and results of tests by historical dataare found to be quite satisfactory.