Estimation of bus travel time is one of crucial issues for passengers to let them know their departure time and arrival time at the destination, and to reduce their waiting time at the bus stop. Furthermore, it can also help to promote the development of city public transportation. To estimate bus travel time, it is worth of knowing whether target routes for passenger's travel is unstable or not, i.e. route with frequent delay or not. In this paper, we present an approach of distinguishing stable and unstable routes using bus probe data. The approach includes two phases; first phase estimates travel time variabilities on routes of a day, and second one distinguishes stable and unstable routes. We divide a day into eight inter-time periods: early morning, morning peak, late morning, midday, early afternoon, afternoon peak, evening, and late night, and detect stable and unstable routes considering the variabilities of standard deviation of the routes in a day. We used bus probe data collected from November 21st to December 20th in 2013. Considering the overall distribution of standard deviation of average travel time of all routes, we defined the criterion to distinguish stable and unstable routes and found the rate of unstable routes on weekends tended to become more than that on weekdays.