It is known that temperature dependence of the photosynthetic rate varies among ecotypes from different latitudes and among those from different altitudes. In general, plants from low temperature regime have the photosynthetic capacity adapted to low temperature; for example, the optimal temperature of photosynthesis is low. However, the biochemical mechanisms of the variation are not fully understood. We examined the temperature dependence of photosynthesis in three ecotypes of Polygonum cuspidatum inhabiting different latitudes and altitudes (Hirosaki, high latitude low altitude ecotype; Fuji, low latitude high altitude ecotype; Tokyo, low latitude low altitude ecotype). Hirosaki and Fuji ecotypes had a lower optimal temperature than Tokyo ecotype. We found significant differences in several biological parameters: temperature dependence of RuBP carboxylation, temperature dependence of RuBP regeneration, and CO₂ concentration in the chloroplasts. In particular, the variation in temperature dependence of RuBP carboxylation was found to be most important. Ecotypes from high altitude and latitude exhibited RuBP carboxylation being more sensitive to temperature: both increase and decrease at low and high temperatures, respectively, were steeper. These changes were responsible to the ecotypic difference in temperature in temperature dependence of photosynthesis.