Thermal rate constants for the N(D-2,P-2) + CH4 (CD4) reactions have been measured using a technique of pulse radiolysis-resonance absorption in the temperature range between 223 and 298 EC. Activation energies determined from the temperature dependence were about 1.5 and 1.0 kcal/mol for the reactions of N(D-2) and N(P-2), respectively. The H/D kinetic isotope effects were about 1.8 and 1.6 for N(D-2) and N(P-2), respectively. The rate constants for N(P-2) + CH4 were much smaller than those for N(D-2) by a factor of 40-60. Variational transition-state theory calculations of the rate constants for the N(D-2) + CH4 (CD4) insertion reaction have been carried out using the reaction path information obtained from ab initio molecular orbital calculations. The comparison between the calculated and experimental rate. constants shows that multiple surface coefficients are larger than the statistical value, meaning that nonadiabatic transitions are important for the N(D-2) + CH4 reaction.