A processing method for image interpolation is provided. In the method, a contour containing a destination pixel and a plurality of contour pixels is produced and detected by using an edge detection method for a selected area in an original image. Next, two extreme points are identified from the contour pixels by judging the changing trend of brightness value of the contour pixels on the contour. Then using the changing trend and two extreme points determines whether the destination pixel is located at a transition area or a non-transition area. If the destination pixel is located at the non-transition area, an interpolation adjust value is calculated according to the brightness values of the two extreme points and the brightness value of the destination pixel. An interpolation result of the destination pixel is obtained by performing the interpolation on the selected area according to the interpolation adjust value.