Organic semiconductor gas sensor is one of the promising candidates of room temperature operated gas sensors with high selectivity. However, for a long time the performance of organic semiconductor sensors, especially for the detection of oxidizing gases, is far behind that of the traditional metal oxide gas sensors. Although intensive attempts have been made to address the problem, the performance and the understanding of the sensing mechanism are still far from sufficient. Herein, an ultrasensitive organic semiconductor NO2 sensor based on 6,13‐bis(triisopropylsilylethynyl)­pentacene (TIPS‐petacene) is reported. The device achieves a sensitivity over 1000%/ppm and fast response/recovery, together with a low limit of detection (LOD) of 20 ppb, all of which reach the level of metal oxide sensors. After a comprehensive analysis on the morphology and electrical properties of the organic films, it is revealed that the ultrahigh performance is largely related to the film charge transport ability, which was less concerned in the studies previously. And the combination of efficient charge transport and low original charge carrier concentration is demonstrated to be an effective access to obtain high performance organic semiconductor gas sensors. An ultrasensitive organic semiconductor NO2 sensor based on crystalline 6,13‐bis(triisopropylsilylethynyl)pentacene films is achieved with a sensitivity over 1000% ppm–1 and fast response/recovery within 200 s/400 s. The relationship between sensor performance and film charge transport is studied. The low original carrier concentration and efficient charge transport are demonstrated to be key factors for the ultrahigh performance.