Rice is one of a world's most important food crops. Rice is exclusively grown in all over the world, in various agro ecosystems. Climatic changes, loss of agricultural lands because of urbanization are some of the critical points in rice production in the world. In this experiment we phenotyped 198 tropical japonica rice accessions for their cardinal temperature (Tbase,Topt and Tmax) with the objective of studying their diversity. Precise cardinal temperatures and knowledge on the diversity is very important in breeding programs and crop improvement. We used a method based on leaf elongation rate (LER) to determine the cardinal temperatures. Leaf elongation rates were determined at six different temperatures; 16, 20, 23, 26, 30 and 35°C. The relationship between LER and temperature followed a beta distribution curve with cardinal temperatures as parameters. Results obtained with high precision presenting a good diversity for the three cardinal temperatures among the japonica accessions we used. Tbase range was between around 8°C and 16°C. The range of optimum temperature was between around 28 °C and 38 °C. The maximum temperature range was between 34 and 55°C. These cardinal temperatures were significant with the varietal type (traditional and improved). The base temperatures were significant with the genetic groups defined by a previous microsatellite study. Main imperfection of this method is that it is a time consuming task. Broaden the temperature ranges at the two extremes will facilitate to have more precise values for Tbase and Tmax. Determine the cardinal temperatures of indica and temperate japonicas will permit to have a better knowledge of the diversity of the all Oryza groups.