Time-resolved microscopic optical second-harmonic generation (TRM-SHG) technique provides a different approach to study the dynamics of carriers in organic field-effect transistor (OFET). Different from many common methods, the TRM SHG directly probes the transient electric-field distribution in the channel of the OFET in high temporal and spatial resolutions. In this work we used this technique to quantitatively study migration of the peak of the electric field. We found that under a broad range of experimental conditions, the migration of the peak of the electric field follows a diffusionlike behavior; namely, the square of the peak position of xmacr is proportional to time t . Based on a two-dimensional computational simulation and general carrier transport mechanism, we proposed that this behavior arises from a simple relation, μ≃γ xmacr 2/(t|Vgs|) , where μ is the carrier mobility, γ is constant about 0.5, and Vgs is the gate voltage with respect to the source voltage. The experimental and simulation data well supported this relationship.