Abstarct: In order to assess the paleostress tensor in the southwest of the Borujerd (north edge high Zagros), 114 fault slip data and with 70 earthquake focal mecanism (related to NW Zagros) have been analysised using inversion technique and Right dihedra method. Initial analysis of the data shows that there is more than one stress phase in the region. Therefore, in the step subsets of homogenous data related to one stress phase, were separated in the heterogeneous data collections, and the stress tensor of each phase was determined. The analysis of data shows strike-slip stress regime whit 026/32, 180/55 and 289/12 orientation for σ1, σ2 , σ3 respectively and stress ration of 0.7 from Pliocene. Also, Compressional stress regime 076/11, 167/07 and 290/7 and orientation σ1, σ2 , σ3 respectively and stress ratio 0.5 before Pliocene. As the study area is located between Dorud and Nahavand faults a localy stress tensional seems to be created. The orientation principal stresses as resulted from rotated data shows orientation to σ1, σ2 and σ3 equal to 202/00, 332/90, 112/00 for strike slip pase and 347/01, 257/02 , 108/88 for compressional phase. In addition to method of analysis of the fault slip data, earthquake focal mechanisms were also used to determine the stress field in Northwest Zagros using inverse method and averaging of P and T orientation. On the basis of average of P&T axes, orientations of σ1, σ2 and σ3 for the high Zagros have been determined to be 357/00, 092/72, 270/21 and in the northwest of folded Zagros to be 030/06, 120/10, 290/80 respectively. On the basis of inverse method the orientations of σ1, σ2 and σ3 stresses in northwest of high Zagros have determined to be 350/04, 103/81 and 260,08 , and in the northwest of folded Zagros have determined as be 039/03, 129/04, 272/85. The stress retion was determined to be 0.1 and 0.8 for high Zagros and folded Zagros respectively. Finally, results of the two methods of averaging and inversion of slip data were compared which are almost identical.