Improving the quality of city bus services is always a very important issue for public transportation managers. How to make the city bus operators to find out the priority of service quality attributes under the limited resource and to achieve the improvement strategy is worth discussing. This study explores the improvement strategy of city bus service quality via a three-dimensional model that is based on Kano's model, importance-performance analysis and goal difficulty. This study took Taipei City Bus as an example for data collection. Through the analysis of Alliance Index, the research result showed that ＂providing services with safe bus＂ ranks first in priority improvement. The second is ＂the waiting time will not be too long＂, and the third is ＂the driver is running smoothly and has good technology.＂ The Alliance Index generates a comprehensive information and offers a strategy for quality improvement. Finally, this study provides some managerial implications according to our results and suggestions for the future research.