Intravascular optical coherence tomography (OCT) has been proven a powerful diagnostic tool for cardiovascular diseases. However, the optical mechanism for the qualitative observations are still absent. We address the fundamental issues that underlie the tissue characterization of OCT images obtained from coronary arteries. For this, we investigate both the attenuation and the backscattering properties of different plaque components of postmortem human cadaver coronary arteries. The artery samples are examined both from lumen surface using a catheter and from transversely cut surface using an OCT microscope, where OCT images could be matched to histology exactly. Light backscattering coefficient μb and attenuation coefficients μt are determined for three basic plaque types based on a single-scattering physical model: calcification (μb=4.9±1.5 mm-1, μt=5.7±1.4 mm-1), fibers (μb=18.4±6.4 mm-1, μt=6.4±1.2 mm-1), and lipid pool (μb=28.1±8.9 mm-1, μt=13.7±4.5 mm-1). Our results not only explain the origins of many qualitative OCT features, but also show that combination of backscattering and attenuation coefficient measurements can be used for contrast enhancing and better tissue characterization.