A naturally fractured carbonate reservoir is chosen for implementing uncertainty assessment on estimation of original oil in place. The probability distribution of uncertain variables is defined in reserve estimation and evaluating probable reserve by using Monte Carlo simulation. There serves are estimated for Karabogaz, Kbb-C member of Karababa and Derdere formations. The reserve estimation in fracture is based on available logs and tests about fracture parameters like density and width. The production data is used for verifying the calculated fracture porosity distributions. The mean of reserve distributions are close to deterministic ones. The sensitivity analysis on input parameters showed that matrix porosity, net thickness, and fracture porosity are significant in Karabogaz and Kbb-C formation reserve estimation while water saturation and fracture porosity are most significant in estimation of Derdere formation reserves.