Qalachwalan-Lesser Zab River receives varieties of domestic and chemical wastes from settlements, recreational and agricultural lands that located around the river. Those possibly affect the capacity of natural self-purification. Water samples have been collected from five different points between Qalachwalan to Qaladza district and sag curve were applied to analyzed water quality parameters for the periods of August 2016 to May 2017. The results showed that the dissolved oxygen values varied from 4.81 mg L^-1 during August to 10.35 mg L^-1 in February, biochemical oxygen demand values ranged from 1.47 mg L^-1 during November to 8 mg L^-1 in March along the river. The statistical analysis between predicted and measured dissolved oxygen and biochemical oxygen demand showed that the less value of root mean square errors was 1.09 mg L^-1 for the month of April and the highest value was 3.92 mg L^-1 for dissolved oxygen during May and the less value for biochemical oxygen demand was 0.77 mg L^-1 for August and the highest value 5.08 mg L^-1 for March. The predicted dissolved oxygen overestimated measured dissolved oxygen by percent bias 12.5% for April and 74.9% for May and the predicted biochemical oxygen demand overestimated measured biochemical oxygen demand as well by 23.4% for August and 98.9 % for February. The purification factor varied from 0.03 to 4.45 for all over the periods. The results, indicating that the river was temporarily polluted and has a poor capacity for self-purify particularly during April that the average factor was 0.14.