To better exploit spatial correlations in a video frame, the HEVC video coding standard has introduced many intra prediction modes and a recursive quadtree-based coding unit (CU) structure. As a result, the complexity of Rate-distortion optimized (RDO) HEVC intra mode selection is significantly higher. Many techniques have been proposed to expedite the intra mode selection process to achieve a good overall trade-off between complexity and RD performance. In this paper, we proposed a fast intra decision algorithm consisting of three parts: calculation reduction, adaptive intra candidate selection, and fast depth decision used early termination. Experiments conducted using the HEVC common test conditions show an average of 61.1% (up to 67.7%) time saving with only 1.03% increase in Bjontegaard delta rate (BD-rate) using the proposed algorithm, out-performing existing state-of-the-art algorithms.