In this work we have examined different characteristics of uni travelling carrier (UTC) photodiode (PD) by mathematical modeling of the device. Based on the basic circuit elements contain in the mathematical model, an equivalent circuit model of UTC-PD is developed. The frequency response and pulse photocurrent response agrees well with reported data. To the authors believe such kind of circuit model of UTC photodiode has not been reported elsewhere. The influences of different parameters and device mounting parasitic effects on frequency response have been investigated. Such simplified circuit model will be helpful in predicting different characteristics of UTC photodiode before fabrication.