Background: Tuberculosis is the most common cause of death among people living with Human Immunodeficiency Virus. The World Health Organization recommends that in countries where HIV prevalence is at least 1% among adults and 5% among Tuberculosis patients, all Tuberculosis patients should be offered HIV testing and all People living with HIV should be screened for Tuberculosis. To increase uptake of HIV testing, it also recommends provider initiated HIV testing and counseling. Objectives: The objective of this study was to assess determinants of HIV testing among Tuberculosis patients on DOTS in East Wollega zone, Ethiopia. Methodology: Institution based case control study design was conducted from January – March 2011. Both quantitative and qualitative approaches were used. Cases were tuberculosis patients who have not accepted HIV testing while the controls were those who have accepted HIV testing. A sample of 270 (90 cases and 180 controls) tuberculosis patients, with a ratio of two controls to one case, was used. Simple random sampling technique was used to recruit study subjects and SPSS Version 17 was used for quantitative analysis while verbatim was used for qualitative data. Result: Tuberculosis patients who have tested for HIV have heard their HIV test result in which 23(12.8%) of them were HIV positive and 157(87.2%) of them were HIV negative. The proportion of HIV infected individual was 10(13.2%) among males and 13(12.5%) among females, while its 19(16.4%) among TB patients from urban and 4(6.2%) among those from rural area. Tuberculosis patients having low knowledge about HIV were more likely to reject HIV testing AOR 3.697 (CI 2.135, 6.402) compared to those who are knowledgeable about HIV. Higher level of stigmatized attitude toward HIV and being from urban were also associated with rejecting HIV testing AOR 3.421(CI 1.870, 6.257) and AOR 5.692(CI 1.930, 16.783) respectively. Conclusion: Efforts to promote HIV testing utilization require effective strategies to reduce stigmatizing beliefs towards HIV and PLWHA among TB patients. Health information dissemination regarding HIV testing and risky sexual behavior reduction should have to be targeted most at risk population like tuberculosis patients.