To study coal pyrolysis behavior at underground coal thermal treatment (UCTT) conditions, a modified CPD (M-CPD) model was developed and evaluated using two scales of experiments as well as two different coals, Utah Sufco and Illinois #6. Compared with the original CPD model, three major aspects were changed, including (1) rewriting the algorithm using MATLAB built-in functions; (2) developing a correlation method between coal elemental composition and the initial functional group population to calculate the effect of cross-linking on the lattice structure; and (3) using a two-activation energy approach for gas generation kinetics modeling. Results predicted by the M-CPD model were compared with those from the CPD model and experiments, and reasons for observed differences between the two models and the experimental data are discussed. The instantaneous devolatilization rates and yields of total volatiles and tars predicted by the M-CPD model were in good agreement with experimental data obtained at UCTT conditions.