Bis(1,2-benzoquinonedioximato)platinum(II) [Pt(bqd)(2)] single crystal shows polymorphism and the polymorphic phase transformation occurs by heat treatment. By transmission electron microscopy, Pt(bqd)(2) is found to grow epitaxially as fine needle-like crystals on alkali halide single crystals that exhibit a polymorphic structure of alpha -form which has a longer Pt-Pt distance than the known alpha -form. The epitaxy is discussed from the point-on-line coincidence. The beta -form crystals transform into the alpha -form by heating at approximately 170 degreesC. Such morphological changes of the beta -form crystals were studied with an atomic force microscope and it is concluded that this phase transformation could be proceeded via a gas phase, but not an ordinary solid-solid transition.