Long waiting times at bus stops and low occupancy of buses were the main problems suffered by bus users and bus operators, respectively. The objectives of this study were to analyze the punctuality of stage bus operation in mixed traffic, passenger's waiting time and to assess the characteristics of bus operation punctuality for various traffic and bus operation conditions. This paper presents the punctuality index and expected average waiting time of stage buses which were operated on a 82.6 km bus route with mixed traffic conditions. The historic GPS data from an on-board survey were used. The results showed that the average punctuality index was 0.29 and the expected average waiting time was 28 minutes at a bus stop or bus station. Based on the punctuality index, by referring to the Transit Capacity and Quality at Service Manual (TCQSM 2003) standard, the bus system service reliability was considered to have LOS B, meaning that vehicles are slightly off headway. The conclusion was that the service quality of stage buses in the mixed traffic can be evaluated based on the punctuality index and expected average waiting time by using on-board survey data. © 2011 WIT Press.