A halftone image generation method used in a system including a quantizer, a subtracter, a post-dithering module and an adder for generating a halftone image is disclosed. First, an original image including original pixels having original pixel values is received by the quantizer and a quantized image including quantized pixels having quantized pixel values according to a predetermined threshold is generated by the quantizer. The quantized pixel values from the original pixel values are subtracted by the subtracter so as to generate quantization error values. A post-dithering process is performed by the post-dithering module to generate dither values by using the quantization error values. The quantized pixel values of the quantized image are added to the dither values by the adder. Finally, the halftone image including halftone pixels having halftone pixel values is generated.