Insulation is one of the most important parts in a high voltage equipment. There are gaseous, liquid and solid insulations which are commonly used. In a high voltage transformer for example the insulating materials are all used. During operation of a high voltage equipment high electric stress may occur. Under extreme condition failure of the insulation may take place. Excessive electric field in air may cause corona discharges while in liquid insulation discharges may take place in the form of streamer. This paper reports experimental results on the corona and streamer discharges in air and silicone oil. The discharges were artificially generated around a needle tip in a needle-plane electrode system with gap length of 4 mm under sinusoidal and triangular voltages. The needle was made of steel with tip radius of 3 μm and curvature angle of 30°. The needle was made by Ogura Jewelry. The discharge pulses were measured using personal-computer based partial discharge (PD) measurement system with sensitivity of better than 0.5 pC. The system is able to measure discharge in time sequential. Phase-resolved analysis of the discharges was done to interpret the physical processes behind the discharges. The experimental results showed that corona discharges took place at negative half cycles. The discharges were concentrated around 270° of phase angle of applied voltage. The discharge magnitude and discharge number of corona clearly dependent on the instantaneous of applied voltage. These were strongly supported by the application of triangular voltage. Streamer discharges occurred at both positive and negative half cycles. The discharges pulses concentrated around the peak of applied voltage at phase angle of 90° and 270°. Experimental results under sinusoidal and triangular voltages revealed that streamer discharge magnitude as well as probability of occurrence was strongly dependent on the instantaneous applied voltage.