The experimental measurements of nonlinear modulated photoreflectance (MPR) phenomena in Si and GaAs wafers, in which the obvious second‐harmonic (2 f) MPR signals are detected in addition to the fundamental frequency (f) MPR signals when the pumping laser power is sufficiently strong, are reported. Meanwhile, a linear‐to‐quadratic transition in the curve of thefMPR signal versus the pumping laser power is observed in Si samples but not in GaAs samples. It is also found that the 2 fMPR signals are more sensitive to the ion‐implantation doses than thefMPR signals which are traditionally applied to monitor the ion implantation in semiconductors. It is found that the contribution of the photoinduced modulation of the surface electric field is the main source of the nonlinear MPR signals and the contribution is theoretically discussed in detail. A one‐dimensional theoretical model is established, and the theoretical results are in good agreement with the experimental data. The dependencies of thefand the 2 fMPR signals on the third‐order optical nonlinear susceptibility &khgr;(3)and the surface‐state densityNsare also discussed, and a method for measuring &khgr;(3)andN4of semiconductors is introduced.