Electrostatic probe methods and optical emission spectroscopy have been used for diagnostics of the low-pressure inductively coupled radio frequency plasma at pressures between 5 and 80 mTorr under real environments of diamond deposition. The single, triple, and emissive probe methods were used alone or combined to obtain electron temperature (T-e) and electron density (n(e)). The single or triple probe method combined with the emissive probe method allowed the determination of n(e) with high reliability as confirmed by cross-checking. Comparison with the grown deposits suggested the existence of a critical n(e) value of around 2 X 10(10) cm(-3), which was responsible fur the drastic decrease in diamond growth rate observed below 10 mTorr and the resulting no growth at around 5 mTorr. The energy distribution of excitation levels of the H atom Balmer series was almost constant and the distribution equilibrium was not established except the case of 5 mTorr, which was ascribed to the difference in the excitation process of H atoms.