A new type of optical-optical double resonance (OODR) experiment using an optical phase-lock loop was applied to a molecular system. The difference frequency between two single-mode lasers was locked to a tunable MW reference using an optical heterodyne technique. An OODR signal was observed in the sub-Doppler condition when three rovibronic levels in the B (1)Pi (u)-X (1)Sigma (+)(g) transitions of the Cs-2 molecule interacted simultaneously with the two lasers. This optical phase-locked OODR spectroscopy achieves absolute frequency measurements of the rotational structure. It is especially advantageous to the ground state of nonpolar molecules because the resonance signal is free from the radiative lifetime of the electronic excited state, and then the accuracy would be as precise as that obtained by MW spectroscopy.