The crystal transformation in lamellar crystals of polybutene-1 grown from an amyl acetate solution was studied by cryogenic high-resolution transmission electron microscopy. The shape of the transformed trigonal (form-1) crystal domains in the surrounding tetragonal (form-2) crystal was successfully revealed. Along with the nucleation of the "untwinned" form-1, the existence of another nucleation mechanism which creates the "twinned" form-1 was suspected. Each crystallite of form-1 was comparatively large, and accordingly, the growth of the form-1 crystal was thought to progress not stem by stem but by pulling in the molecular chains from the surrounding form-2, creating new stems of form-1. Once form-1 has nucleated, mutual orientation between the form-1 and the form-2 domains appeared to be insignificant for further growth. The shape of the form-1 domain was irregular, and no specific crystallographic direction along which the form-1 domain tends to grow was found.