: Diagnosis of brucellosis is based on the rises of antibody titer in patient sera. It is reported clinicians are frequently complain on some of serology results of brucellosis. In this research, all brucellosis patients who were confirmed clinically and by serological methods are entered to the study. In total, sera of 758 patients of Hamedan teaching hospitals were analyzed with Wright, Coombs and 2Mercapto- Ethanol tests. All these patients were received treatment after final diagnosis. In the studied group 29 (3.82%) and 30 (3.95%) of patients had no antibody rise in Wright and 2ME respectively. 0ne patient had antibody titer less than 1/40 in Coombs test. Specific antibody level higher than 1/160 was observed in 49.31% of patients for the Wright test, while it was 43.46% in patients with antibody titer higher than 1/80 of 2ME test. Conclusion: It seems some of those serology negative patients may involved with brucellosis and possibility of infection should be always considered in those serology negative patients since B. abortus antigen are applied in all serology tests. Analyzed results also indicate application of Coombs test can provide clear understanding for those cases with low antibody rises.