An image compression/decompression device and a method thereof are provided. In the method, a first array is generated according to an input image array, and further, a second array is generated by performing an edge detection algorithm according to the input image array. Besides, a plurality of elements of a third array is generated according to the first array. Furthermore, a group of elements of the third array is selected according to the second array. In addition, an average value of the group of elements is computed to obtain an element of an output image array. Each element of the input image array and the third array is M bits, and each element of the first array is N bits, wherein M and N are natural numbers, and M is larger than N. Thereby, the quality of a decompressed image is improved.