Current traffic microsimulation tools are usually of limit use for evaluating the operations of pedestrian-involved systems, especially under unsignalized conditions (e.g. jaywalking and mid-block pedestrian crossings are not typically modeled), due to insufficient understandings of Pedestrian-Vehicle Interaction (PVI) behavior at such locations. This paper studies pedestrians' gap acceptance behavior when they jaywalk outside crossing facilities, which is regarded as the most basic and important part to improve current PVI behavior models. A field study was conducted to collect the data of pedestrians' decisions and concurrent traffic status using a set of synchronized video cameras on a typical unsignalized urban road section. The data were used to develop and validate a pedestrian gap acceptance model based on discrete choice approach. The initial results with the model are appealing when considered in combination of simplicity and accuracy. The results can be used to supplement existing guidelines for pedestrian-involved problems, or to form a knowledge base to incorporate pedestrians into current vehicle-dominated microsimulation models in a more realistic way.