Nanoelectromechanical resonators are promising as filters and sensors due to high Q factor. However, effects of nonlinearity become more prominent as the resonator dimension is reduced. In this work, room-temperature experimental data of Al/SiC nanoresonators obtained using magnetomotive transduction is analyzed with a dynamic model including nonlinear spring and damping effects. Transition between the linear harmonic oscillation and nonlinear regime is observed. Nonlinearity caused by large AC driving voltage is reduced with increasing DC voltage, and vanishes when the resonant amplitude becomes smaller than the calculated critical amplitude.