An image processing method is provided. A digital image is outputted by an optical sensor and includes M pixels. The ith pixel among the M pixels has an ith set of original gray levels, wherein i is an integer index ranging from 1 to M. In the method according to this invention, M sets of logarithm values are first generated. The ith set of logarithm values among the M sets of logarithm values is generated based on the ith set of original gray levels and a set of adjusting parameters relative to the optical sensor. Statistics of the M sets of logarithm values is then gathered. According to the statistics result, a target color temperature of the digital image is determined.