Bus services usually tend to be irregular and their level of irregularity depends on various factors, such as crowding level, terminal departure behaviour, passengers’ behaviour, operator behaviour, traffic and weather condition and etc. High-frequency bus routes have shorter headways (usually headway less than 10 to 15 minutes) and higher passenger demand compared with normal routes. Therefore, level of irregularity can be even higher in bus service at high-frequency operation. Running time variability comes from both systematic changes in ridership and traffic levels at different times of the day, which can be accounted for in service planning, and the inherent stochasticity of homogeneous periods, which must be dealt with through real-time operations control. This study evaluated impact of ridership changes and traffic condition through time of the day on running time variability, using Automatic Vehicle Location system (AVL) and Automatic Fare Collection system (AFC). All data extracted and collected from RapidKL Company for route U32, which is a high-frequency route in downtown of Kuala Lumpur. Descriptive analysis on data showed a high variation in running times, especially in morning peak hours. A liner regression model also proved than crowding level (extracted from AFC data), number of stops and congestion zones have relatively high impact on running time variation.