The traditional non-contact electro-optic (EO) probing technique is further extended to measure quasi-dc bias voltages. We propose a special external LiTaO3 probe equipped with a grounded side-electrode (GSE) and a power supply modulation (PSM) method. The GSE can improve the voltage measurement sensitivity and error due to the tip position variation. The PSM can make all bias voltages chopped and readily picked up by using a high sensitivity lock-in amplifier. Test results on model circuits demonstrated that the proposed new EO probing system can obtain voltage measurement results that are in good agreement with known values. With this extension function this new system can perform ac high frequency and dc bias voltage measurements as well.