We present mixed quantum-classical (QC) study of A+CH4 reaction with A=H, D, and T using the semirigid vibrating rotor target (SVRT) model to investigate the mass effect of isotope reactions by comparing with the results from quantum SVRT calculations. In this mixed quantum-classical (QC-SVRT) approach, the relative translational motion between the atom and the molecule is treated by classical mechanics while the rest of the coordinates are treated quantum mechanically. The reaction probabilities and rate constants of three isotopic reactions are calculated on the potential energy surface of Jordan and Gilbert. Our study shows that the mixed QC-SVRT reaction probabilities and rate constants from the ground vibrational state of the reagent become progressively more accurate as the mass of the projectile is increased. This phenomenon is related to the diminishing role of the zero-point energy of the projectile as the reduced translational mass is increased. Our study also finds that the QC-SVRT results from vibrationally excited reagent are in substantial deviations from the quantum SVRT results which is due to the failure of the single configuration nature of the mixed quantum-classical approach. (C) 2003 American Institute of Physics.