This paper proposes a new method to improve the performance of intra-field interpolation which is the most important element of deinterlacing method. Traditional edge- based methods were poor at determining accurate edge direction which decides interpolation performance. The proposed method decides the direction and slope of edge based on locally adaptive-thresholded binary image to improve efficiency and accuracy, and it also increased the horizontal size of search window to 15 for better direction determination at gentle slope. Compared to traditional edge- based methods, interpolation based on binary image, which is generated using the proposed method, leads to great performance improvement. With computer simulations for a variety of images, this paper shows the new approach is much better than traditional methods.