Electric field distributions of resonant modes in a photonic crystal nanocavity were imaged using atomic force microscope nano-oxidation. A grid pattern of nano-size oxides was grown on the nanocavity to perturb the resonant modes. The perturbation caused a shift in the resonant wavelength that was proportional to the local electric field intensity of the resonant mode. The experimentally obtained field intensity images agreed excellently with the finite-difference time-domain calculations. The measured resonant mode images had high spatial resolution and image contrast, owing to the extremely local perturbation of the atomic force microscope oxidation technique.