Finding new antimicrobial agents from natural compounds with less side effects has been considered by number of researchers in the world. It is important to achieve efficient and up-to-date results in order to identify a substance with antimicrobial properties and achieve operational methods to increase these traits in a society where the prevalence of various diseases has been increased. In fact, the purpose of this study was to achieve compounds from microalgae with antimicrobial properties to be used in food and pharmaceutical industries that can have good consequences for human health. Therefore, in the current study antimicrobial activity of ethanolic extract of microalgae Chlorella sp., that was cultivated under different light conditions, was investigated. For this purpose, microalgae Chlorella sp. was separately cultivated under red, blue, green and white lights with intensity of 109 (μmol-photon m-2 s-1) and antimicrobial potential of the microalgae extracts investigated against the activity of Streptococcus mutans. In addition, Minimal Inhibitory Concentration (MIC) of the extracts determined. Based on the results, the wet extracts indicated more average antimicrobial activity than dried ones. Furthermore, the wet extract of microalgae cultivated under the red light showed a stronger antimicrobial activity compared to extract obtained under the other light spectrum with the minimum inhibitory of that was 10 mg/ml. Also, extract obtained under white light had no significant antibacterial activity against the bacterial strain.