Obvious heating effects have been observed in RF heating experiments on CT-6B tokamak, in which microwave beams of frequencies 34.34 GHz and 20.1 GHz generated by two gyrotron systems were launched from top and outside of the device, respectively. It is shown that these heating effects arise becouse of the different heating mechanisms. The former is due mainly to the resonance heating of O mode. The latter belongs to down shift-over dense heating. Its heating mechanism may be the nonlinear phenomena occuring near the high density cut off layer of X mode. A kind of nonlinear heating phenomena (secondary heating) has been observed in two experiments, which may come from the different physical mechanisms.