The crystal growth of isotactic polystyrene (it-PS) is investigated in very thin, 11 nm thick films. The it-PS crystals grown in the thin show quite different morphology from that in the bulk. With decreasing crystallization temperature, the branching morphology in a field appears: dendrites and compact seaweed. The branching morphology is formed through a morphological instability caused by gradient of film thickness around a crystal; the thicker the film thickness, the larger is the lateral growth rate of crystals. Regardless of morphological change, the growth rate as well as the lamellar thickness depends on the crystallization temperature as predicted by the kinetics.