Wheat (Triticum aestivum L.) and rice (Oryza sativa L.), which belong to the Poaceae family, are starch-grain crops, but wheat is adapted to cooler temperature conditions than rice. In this study, the difference between the two contrasting crops in grain-filling adaptability in response to temperature was investigated. Two spring wheat cultivars were grown in the Mediterranean-type environments of Western Australia and Southeast Turkey, and four temperate-zone rice cultivars were grown in several locations in Japan under irrigated conditions. Portions of the crops were enclosed under a plastic canopy to elevate the temperature after anthesis. Average temperatures during grain filling ranged from 14 to 24 °C for wheat and from 23 to 29 °C for rice. Grain yield varied from 280 to 599 g m−2 in wheat and 354 to 736 g m−2 in rice. When plant density was halved at flowering to estimate the potential grain-filling rate under an increased supply of assimilates, the grain-filling percentages [%F, observed grain weight (G)/potential grain weight (PG)] of both crops were represented by similar logistic curves of cumulative temperatures during the grain filling period. These results suggest that grain-filling responses to temperature scarcely differ between spring wheat and temperate-zone rice. G was estimated for the spring wheat and temperate-zone rice cultivars under different temperatures after anthesis using an assimilate-limited grain-potentiality model consisting of the following parameters: rate of whole-plant weight increase (ΔW/Δt), rate of potential grain dry weight increase (ΔPG/Δt) based on rate of%F (%F/Δt) and PG, and the amount of stem reserves (SP). The observed data showed that the decrease in ΔW/Δt with an increase in temperature in wheat was greater than in rice. According to the model, G started to decrease at lower temperatures in wheat than in rice, and this decrease was accelerated by lower amounts of SP. Therefore, the difference in the optimal temperatures for G during grain filling between the two crops was suggested to mainly result from the sensitivity of assimilation to high temperatures.