A new measurement method is proposed to separately measure the effect of ink spread and optical dot gain on the MTF (modulation transfer function) of printed images from an ink jet printer. The transmittance and reflectance of the same printed images are captured with two different illuminations: one on the printing surface (for measuring reflectance) and the other through the paper base surface (for measuring transmittance). The MTF of ink image and MTF of print are measured from one-pixel line printed on glossy, matte and uncoated paper. The result shows that the paper type significantly affects the MTF of ink image and the MTF of print. The MTF of paper is also measured by this method and it is shown that optical dot gain considerably affects the MTF of print.