This paper explores varieties of rice as one of the fundamental determinants of rice harvest in Taiwan. Specificaly it examines, as a case study, the evolutionary process in a transition of Taiwanese rice from subsp. indica to subsp. japonica, a process manifested by change in spatial distribution of the major rice species in the latest 15 years. Through field work and library research, we identified six phases for the transition of Taiwanese rice:from the natural growth phase through transplanting by indigenous people, Dutch, Han Chinese and Japanese to the recent periods of planned species improvement. We also traced the diffusion process to the ecological adaptation of planned species. Threes diffusion patterns since 1980 have been clear: one being island wide, one along particular direction from the original location of cultivation, and the third to adjacent districts. New trends have appeared lately, however, within each diffusion pattern, certain species have succeeded the others. Both environment and economical factors explain.