Dental caries is an irreversible microbial disease of the calcified tissues of the teeth. Streptococcus mutans is a bacterial resident of the oral cavity and is considered to be the principal etiological agent of dental caries in humans. Therefore, the aim of this study was to determine the prevalence and risk factors associated with streptococcus mutans dental caries. A cross sectional study was conducted among patients who attendeded Jimma University specialized hospital detal clinic. patintes demographic and clinical information was collected by using pre-tested questionnaire Dental plaques from all patients was picked up by forceps and suspended in to phosphate-buffered saline for further streptococcus mutans identification. From a total of 115 study participants 56(48.7%) and 59(51.3%) were males and females respectively. The overall prevalence of streptococcus mutanswas 79(68.7%) among patients with dental carries. Twenty-five (21.7%) participants had gem bleeding and 47 (40.9%) participants had previous tooth decay. In this study, streptococcus mutans is the common public health problem among dental carries patients. Taking soft drinks, oral debris and gingival index were risk factor for dental caries of streptococcus mutans.