A large parts of home-made military aircrafts are developed through imitation and modification; their R&D cost can not reflect the truth when they are developed as a new type. So, these data can not be used directly in the study of R&D cost for a new type. In order to apply these data reasonably, a modification function is defined based on the statistic data of 17 homemade aircraft types. Then, according to the statistic data of home-made aircrafts, the parameter's value of the modification function is evaluated and the modified R&D cost for the Imitated and Modified Aircrafts is calculated. Finally, the results are validated by using the Partial Least - Squares Regression (PLSR).