We demonstrate bifurcation diagrams in entrained state on the Oregonatore system under sinusoidal perturbations with the 2nd harmonics. The diagram against the amplitude of 2nd harmonics shows the various shape depending on the perturbed frequency, &ohgr;, and the initial phase, &Jgr;, of the 2nd harmonics. The numerical results were well agree with theoretical ones based on the phase dynamics theory. ©2000 American Institute of Physics.