Thermal development of photoresist films in lithography is an interesting alternative to the common wet development technique, which can result in problems such as swelling, line edge roughness, and pattern collapse. The authors investigated thermal development of a nonchemically amplified calixarene type high-resolution electron beam resist. Isolated and dense structures were developed at 250-400 degrees C in air and in low vacuum conditions. They achieved 25 nm half-pitch dense line/space patterns and 17 nm isolated lines without sacrificing resist resolution, sensitivity, or contrast. The obtained lines were smoother with more than two times smaller line edge roughness. The results demonstrate the feasibility of using thermal development for high-resolution nanolithography.