1. A method of determining a motion vector of a target image block, comprising: performing motion estimation upon the target image block and a plurality of reference image blocks to calculate a plurality of block matching differences; determining a minimum block matching difference from the plurality of the block matching differences; comparing the minimum block matching difference with at least one of the reference block matching differences to obtain at least a comparison result; and according to the at least a comparison result, selectively determining a target motion vector of the target image block referred to a motion vector of at least a neighboring image block of the target image block or a motion vector corresponding to the minimum block matching difference wherein when the comparison result shows that the difference of the minimum block matching difference and the reference block matching difference is not greater than a second reference value, a decision circuit determines to utilize the motion vector of at least a neighboring image block to generate the target motion vector of the target image block, else determines to utilize the motion vector corresponding to the minimum block matching difference as the target motion vector of the target image block; wherein the method is used in motion interpolation for frame rate conversion; the reference image blocks are located outside a first range corresponding to the reference image block of the minimum block matching difference, a first reference value is determined by the maximum block matching difference and the minimum block matching difference of the plurality of block matching differences, and the first reference value is used for defining a position of an image block included by the first range.