Service coverage of a public transportation means by definition is the area that is reasonably walkble by the customers, or they are willing to use the transit. It is one of the most important criteria to evaluate the performance of a public transit in term of availability or "willingness to serve". In order to design a good transit system, its service coverage must be at first carefully considered and selected in accompany with of the best service frequency and schedule adherence performance. This paper presents some research results of a sequence procedure to select, calibrate and validate the variables to define service coverage for the bus transit in the context of the urban areas of Vietnam.