Abstract: On the basis of probability method,the present authors obtain the equations for describing the sol-gel distribution behavior and the critical condition for gelation of A_a-B_b,C_ctype copolycondensation system. With the aid of equations mentioned above,we also obtain the equations for describing the limit-curves of curing region.The equation for lower-limit curve of curing region is given byand the equations for upper limit-curve of the curing region are given by (a-1)(b-1)r_b+(a-1)(c-1)r_c=1 and the equations for upper limit curve of the curing region are given by   Where n_b=(a-1)(b-1) and K is a parameter in relation to the relative reactivities of B an d C groups and r_band r_cin turn denote the stoichiometric ratio of Bto A and C to Agroups respectively.The copolycondensation of adipic acid butylene glycol-trimethylol propane has been carried out with different r_band r_cvalues and the corresponding gelation phenomena have been observed.The theoretical limit-curves of curing region and the observed values are nearly in coincidence with each other.