Optical techniques based on measuring scattering properties of bio-tissues have a potential to determine internal physiological state. For tissue imaging,the least-scattered photons arriving at a detector may have a significant effect on the development of imaging algorithms. Tracing least-scattered photons may be helpful for further understanding of imaging mechanisms within current high-resolution optical-imaging techniques. Based on the vertex-propagator,a coordinate-tracing model is developed to search probabilities for least-scattered photons traversing in a scattering medium. Integral expressions for photon experiencing any time scattering events are obtained. Compared with the methods based on both geometrical tracing and Monte Carlo simulation,our model is simpler and easier for programming.