I-V characteristics of Josephson junction (JJ) under microwave irradiation is simulated taking the resistance, capacitance and inductance of the junction (RCLSJ) into account. Since most of the JJ made with high TC material operates at high temperature where the thermal fluctuation is playing significant role. In the present simulation, thermal noise is also taken into consideration and simulated result explains the rounding behaviour of the Shapiro step observed in the experimental I-V curve. The simulated results also confirm the growing of Shapiro steps with increasing microwave power. This model also clearly account for the appearance of broad minima and reduction of the maxima in the power dependence of the step amplitude ΔIn/2Ic as observed in the experimental data of Kautz et al. (1992).