Converting rice straw or rice husk into biochars is one of the most effective ways to reuse them. This study examined the effect of rice variety and pyrolysis temperature on the properties of biochars, produced from rice straw and rice husk of Koshihikari (a typical rice variety of Japan) and IR50404 (a typical rice variety of Vietnam), in the temperature range from 300 to 800 °C. Biochars produced at high pyrolysis temperatures (> 500 °C) showed higher surface area (approximately 3 times) and higher Si content (by more than 15%), but lower H/C and O/C ratios in comparison with biochars produced at lower temperature. With regard to rice variety, Japanese Koshihikari biochars possessed higher Si content (almost 20%), but lower specific surface area and O/C and H/C ratios than Vietnamese IR50404 rice residue biochars. The surface area of Vietnamese-rice-straw biochars at 600 and 700 °C was ~ 30% more than Japanese-rice-straw biochars, while the surface area of Vietnamese-rice-husk biochars was slightly more than that of Japanese-rice-husk biochars. Higher Si content in Japanese Koshihikari biochars was predicted as one of the main reasons for the lower surface area in their biochars, due to the higher possibility of pore-filling or blocking by silica.