In this paper, we present a frame rate up-conversion method for ultrasound image enhancement. The inherent flexibility of ultrasound imaging and moderate cost without known bio-effects give ultrasound a vital role in the diagnostic process compared with other methods. The conventional mechanical scan method for multi-planar images has a slow frame rate. In the proposed frame rate-up conversion method, new interpolated frames are inserted between two input frames, giving smooth renditions to human eyes. Existing methods employing blockwise motion estimation show block artifacts, in which motion vectors are estimated using a block-matching algorithm (BMA). We propose an optical flow based method to find pixelwise intensity changes that yields more accurate motion estimates for frame interpolation. Consequently, the proposed method can provide detailed and improved images without block artifacts. Interpolated frames may contain hole or overlapped regions due to covered or uncovered areas in motion compensation. Those regions can be easily eliminated by a post processing, in which the similarity of pixel intensity is employed with a ray casting based method. Experimental results with several sets of ultrasound image sequences show the effectiveness of the proposed method.