The effect of spatial resolution ratio on the quality of optical remote sensing image fusion was discussed. By simulating images with continuous resolution ratio from IKONOS-2 panchromatic and multispectral images, experiments were carried out on Gram-Schmidt pan-sharpening method provided by ENVI V4.4, which could be seen as a complement of existing achievements. Results show that fusion quality deteriorates as the spatial resolution ratio decreases so that the spatial resolution of multispectral should be as high as possible in applications, and that the panchromatic image will need to be down-sampled to perform better fusion quality if the spatial resolution ratio is too small. Furthermore, fusion results have better quality than their original multispectral images even at the situation of a very small spatial resolution ratio.