One informal public transport service in Bangkok is Silor (SR), given the meaning in Thai as four-wheeler. SR facilitates urban mobility both in terms of major travel mode and feeder to bus and mass transit lines in the city. This research aims to explore user subgroups characterized by attitudes through an Exploratory Factor Analysis followed by commuter segmentation through Cluster Analysis based on the obtained latent variables in order to propose future implications for SR service improvement in Bangkok. Interviews are conducted through questionnaire survey to collect users' socioeconomic and trip profiles and attitudes towards SR service. The result reveals dimensions of service quality, which explain user perception and priorities regarding the service. Four segments of SR users are identified by four attitudinal factors, including reliability, in-vehicle environment, comfort and convenience, and environmental impact. Personal profiles, trip characteristics, and attitudes of each segment are analyzed and compared. Research findings would contribute to new knowledge on quality factors and area of SR service improvement as well as provide better understanding on diverse perception among SR user segments. Policy implications that best suit for each user segments are discussed.