Non‐thermal effects of millimeter wave (MMW) on Pheochromocytoma (PC12) were studied by potential measurement with a voltage sensitive dye (DiBAC4(3)). Cells were irradiated at fixed frequencies of 30, 40, 60, 76GHz as well as sweeping frequency between 10 and 100 GHz by an MMW generator based on a uni‐traveling‐carrier photodiode (UTC‐PD), the most widely tunable MMW source. However there were no significant changes in membrane potential between MMW‐irradiated and control cells. The results suggest that MMW irradiation in the range from 10 to 100GHz appears to be safe for ordinary PC12 cells under non‐thermal conditions. © 2004 American Institute of Physics