The corona effect in the power system has a dominant role in reducing the efficiency of the high-voltage lines. In this study, the current and voltage impulses, which are important in corona determination, have been examined in frequecy spectrogram base from test result. The frequency spectrogram graphs have been obtained from the current-time and voltage-time variation by Matlab program. With these graphs, frequency values have been found to provide an important clarification of the insulation performance of electrical equipment for reliable and accurate diagnosis. These values have been observed according to different conditions like pressure, polarity, and insulation ambient. This spectrogram analysis can be used to find the characteristic frequencies and eliminate the disturbance effect. The time between corona steps decrease when Sulphur hexafluoride (SF6) gas pressure is increased. The time of second corona is close to leader's discharge time. Corona starts early when SF6 amount is decreased in gas mixtures. The corona currents are large in the low SF6 gas mixtures, this situation is related to the high insulation of the SF6 gas. A feature which is dependent the frequency is not found.