Transmission line insulators are one of the most vital equipment in the arena of power system. Owing to atmospheric degradation, insulator gets contaminated. Contamination severity results in the increment of leakage current in presence of moisture. Ultimately it leads towards flashover. For averting flashover, assessment of leakage current is very important. For measuring leakage current, a test setup had been arranged at laboratory. Leakage current data has been investigated using FFT analysis. Here, a new parameter, ratio of boundary frequency to fundamental frequency of leakage current has been investigated extensively. This ratio is mainly affected by humidity as well as by the severity of contamination or pollution level. Analysis of this ratio will be helpful to assess the surface state of the insulator and ultimately flashover can be prevented.