Abstract: By Monte Carlo method, the paper simulates the C/O responses of different spacing in four conditions such as water or NGH in the well, and water or NGH outside of the well, and obtains the different rates of C/O with different spacing for the same formation, different outside material and the same outside material, different formation after data processed. By comparing and analyzing the calculation results mentioned above, the paper concludes that the responses of C/O log are largely dependent on spacing greatly and obtains the optimal range of the short spacing and the long spacing.