We report experimentally how the wavelength dependence affects the OCT imaging contrast and depth by the use of normal human and animal tissues in vitro. Two systems, using the light sources with central wavelengths at 820nm and 1310nm, respectively, were set-up and used in present work to study the light penetrating and imaging contrast dependence on wavelength. The tissue specimens used were colonic and stomach mucosa. The results confirmed that the longer wavelength suffers less scattering and absorption, thus penetrating deeper into the tissue, but pays a penalty of reduced imaging contrast. In general, both imaging depth as well as tissue contrast is determined by tissue absorption, scattering properties and the refractive indices in microscopic and macroscopic scale, which vary with wavelength. Thus, the choice of wavelengths used in the OCT system should be careful in terms of optimizing imaging depth, imaging contrast as well as differentiation between different tissue morphologies.