According to the hypothesis of isomorphy rate, all the lower developmental thresholds of different developmental stages of an insect are equal. However, there is lack of a formal statistical method for testing whether there is a significant difference among the lower developmental thresholds on the basis of the traditional linear model describing developmental rate as a linear function of temperature. For comparing the lower developmental thresholds of different developmental stages, a new method based on the Chow test is proposed in the current study. Another feasible way based on the linear model proposed by Ikemoto and Takai is also proposed. The lower developmental thresholds can be compared by the analysis of covariance on this linear model. The current study can be used to test the hypothesis of isomorphy rate. When comparing the lower developmental thresholds of different geographical populations for one insect species, the two methods proposed here are also applicable.