Although the past research results have provided many effective tools for evaluating public bus service quality, they still lack the concept of risk. This study assessed the quality risk for city bus service by constructing a quality risk assessment model that integrated Kano’s model, IPA, and FMEA. The online questionnaire was distributed to commuters who use BMTA’s bus service in Bangkok with 429 samples. This study analyzed the service quality attributes of bus services via Kano’s model, an IPA, and an FMEA. For the results of Kano’s model, this study identified nine one-dimensional and three attractive attributes. The results of the IPA showed that the safety of boarding and disembarking from the bus, the cleanliness of vehicles, and the punctuality of buses according to the schedule were the top three attributes that had a large gap between importance and satisfaction. According to the results of the FMEA, the safety of boarding and disembarking from the bus, the cleanliness of vehicles, and the punctuality of buses according to the schedule were the top three attributes that had a high service quality risk and considered a priority improvement. These attributes belonged to interaction with passengers, tangible service equipment, and operating management support dimensions. Therefore, the government should take action and improve service quality to make Bangkok commuters consider switching from driving cars to using a bus. Finally, this study provides practical improvement strategies for managers to improve passenger experiences and enhance the quality of bus services.