In this article,the contrast of the images of leaves scanned by THz time domain spectroscopy is discussed.From the leaf′s transmittance images reconstructed at 8 different THz frequencies,it shows the absorption properties of the sample′s different parts are distinct which give out different image contrasts.Better image contrast will lead to clearer picture.The study demonstrates that to obtain better image contrast,it is better to select the frequency around which the transmission amplitude is moderate and the some part of the sample has an absorption peak within this range.