Informal transportation services are often associated with safety risks, as the use of old vehicles in poor condition does not guarantee the safety and comfort of users. Currently, existing public transportation services in Kota Kinabalu does not cover the entire city area, which has led to the existence of informal public transportation mode, namely unpermitted busses to meet users’ demand. The purpose of this study was to assess user acceptance of unpermitted bus services in Kota Kinabalu, Sabah. A survey was done using a questionnaire as an instrument among 140 respondents. The results show that three factors of service effectiveness achieved a high mean score, namely the fare factor (3.82), the reliability factor (3.75), and the accessibility factor (3.69). This study shows that unpermitted bus services play an important role, especially in providing mobility services to users, although drivers do not emphasize the safety and comfort aspects of this mode.