Drug release mechanism of in situ-forming implants (ISIs) based on poly(lactic acid-co-glycolic acid) (PLGA) loaded with leuprolide acetate/-cyclodextrin (LA/-CD) complexes via fitting with four diffusion-based semi-empirical models were studied. The release rate constants and release exponent of ISIs were calculated. The main drug release mechanism was Fickian diffusion. The LA diffusion coefficient and release constant were decreased via increasing the portion of -CD in complexes. The release curve was parabolic, with a higher initial slope and then consistent with the exponential. All ISIs containing LA/-CD complexes better fitted with the Korsmeyer-Peppas, Weibull and Peppas-Sahlin models rather than first-order model. Furthermore, the effect of LA/-CD complexation on the degradation of ISIs was studied through scanning electron microscopy (SEM). Results showed that hydrophilic nature of -CD facilitated the surface erosion of PLGA chains, however after 18 d, ISI-1/10 had still a proper structural strength, due to no hydrolytic degradation of -CD in this implant.