Video coding standards mainly aim to decrease bit-rate to be transmitted while maintaining video quality at a certain level. Mode decision used in inter- and intra-prediction stages is a vital component of video coding and is closely related to video coding efficiency and complexity. Therefore, fast and efficient mode decision algorithms are required to reduce encoding time while keeping the video quality at a certain level. In this paper, a fast inter mode decision method for HEVC is proposed. Firstly, an early selection considering intra-block similarity by making use of integral image at the prediction unit level is introduced. Secondly, a flexible early termination mode at the coding unit level is presented. The performance of the proposed method in these units is evaluated separately and together. Experimental results demonstrate the low complexity of the proposed method without any important loss in coding efficiency. The proposed method provides a good balance between coding efficiency and time savings compared to the state-of-the-art approaches.