The atmospheric pressure radio frequency (RF) inductively coupled thermal plasma (ICTP) has been extensively used for many industrial processes. In order to understand the physical-chemical mechanism involved in the discharge process of ICTP, in situ optical emission spectroscopy (OES) was carried out to diagnose and determine the active particles and electron excitation temperature in this plasma. Several active particles such as Ar*, H-alpha, and H-beta were detected in the emission spectrum of Ar-H-2 ICTP. Based on the Boltzmann plot method, the electron excitation temperature and thermal efficiency of ICTP were evaluated. It was obtained that the electron excitation temperatures in Ar-H-2 ICTP varied from 9651.70 to 16691.91 K when the applied power was in the range of 8-15 kW, which was significantly higher than the electron excitation temperature in Ar ICTP at the same applied power. Besides, the thermal efficiency was enhanced from 17.19% for the Ar ICTP to 30.69% for the Ar-H-2 ICTP. These results may be beneficial for understanding of the discharge process in atmospheric pressure Ar-H-2 ICTP.