Abstarct: Nowadays by increasing the demand inter urban trips as well as increasing The number of vehicles in urban areas, will increase Traffic congestion, increase the Time for inter urban Trips and increasing the Leval of service. Therefore, the intersections capacity Parameter and the Intersection volume parameter are most Important Parameter for designing the Intersection without congestion . One of the most Important Parameter for calculation of intersection capacity is Passenger car equivalent of heavy vehicles. In This Study we aim to calculate passenger car equivalent of inter urban bus for undersaturated and oversaturated signalized intersections of Iran,Mashhad. an other purpose of This study is finding the effect of the variation of volume, capacity and g/C on the passenger car equivalent of bus. Therefor the passenger car equivalent of buses for four under saturated intersection and three over saturated intersection baxsed on delay parameter was calculated using the field data and finally the passenger car equivalent of buses in over saturated regim and undersaturated regim were compared. The field data also were implemented in the traffic software,SYNCHRO and the passenger car of the inter urban buses were calculated with software outputs(delay). Also after determining the PCE by use of field data, in order to generalize PCE into the other intersections in Mashhad, different traffic conditions (g/C=0.25, 0.3, 0.35, 0.4) were implemented in the software and then by use of software outputs(delay in baxse flow and Mix flow) the passenger car equivalent were calculated. In the end two types of mathematical models were presented to determine the passenger car equivalent. The parameters of models are the volume of passing through the intersection, percentage of the bus passing through the intersection and the parameter g/C. finally the capacity reduction factors calculated by passenger car equivalent of buses in this study were compared with the capacity reduction factor calculated by passenger car equivalent of heavy vehicles in HCM2016.