Abstract: In this paper,the functional relation between number of colored vessel and vessel length is derived from a theoretical model.It has been given the theoretical basis of forced paint method that used to study vessel length distribution.The vessel length distribution regularity is derived from a theoretical model.The stems of Cinnamomum camphora (L.)Paesl and Firiana simplex are employed with the forced paint method described by Zimmermann et al .The results from experiment are the same as the result from theoretical model.This shows that the results and conclusion are reliable.