Photoconductive Antenna(PCA) is considered to be very simple and compact Terahertz (THz) generation source based on opto-electrical conversion method. In recent years, several experimental and simulation studies have been performed to analyze a PCA behavior. However, an accurate equivalent electrical circuit model is needed for theoretical analysis of a PCA. As the electrode gap region of PCA can be modeled as a charged capacitor, this work deals with the calculation of time dependent capacitance of a pulsed PCA system. The analysis of time dependent capacitance provides a better understanding of carrier dynamics in PCA and also helps in creating an accurate equivalent electrical circuit of PCA. © 2015 IEEE.