We have studied negatively charged nitrogen vacancy centers (NV- centers) in diamond using the technique of optically detected magnetic resonance (ODMR). Due to the use of the MW power amplifier we have easily observed in the ODMR spectrum all the lines known to date, including the lines of the excited state of the NV- centers. The g values and the hyperfine interaction constants of the ground and the excited triplet states of the NV- center are given. Hamiltonian parameters of the ground and the excited triplet states of the NV- center at ambient temperature have been confirmed and refined. In addition, an evidence of a slight g value asymmetry of the excited state of the NV- center has been obtained.