The popular Structural SIMilarity (SSIM) index has shown to be a good perceptual criterion for testing and optimizing video encoders such as the MPEG-H/H.265 High Efficiency Video Coding (HEVC). However, it is still unclear how to compare two HEVC encoders with a number of bit rates and SSIM values. In this work, we study the video quality comparison of HEVC encoders based on the Bit Rate-SSIM (R-S) curves. Our contributions are three-fold. First, we exploit how to interpolate an R-S curve when only a small number of R-S samples are available. We also develop to minimize the weighted sum of squared errors during the interpolation process, where the weights are pre-defined by users to satisfy the diversified requirements. Second, we propose an approach to evaluate the perceptual coding gain in terms of the R-S improvement between two individual video encoders. Third, as an application, we utilize the proposed approach to compare the R-S performances of HEVC encoders with different configurations.