There is no report on the antimicrobial effects of evening primrose (Oenothera biennis L.) and borage (Echium amoenum Fisch. & C.A.Mey.). In this research, the seeds of these plants were milled and extracted by chloroform:methanol (2:1). Then, the solvents were evaporated under reduced pressure and temperature to extract the oils. Antimicrobial effects of various concentrations of the oils (10- 1000 mg/l) were assessed against Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, Candida albicans and Aspergillus niger. Minimum inhibition concentration of the oils for each strain was measured using Mueller Hinton Broth and Sabouraud Dextrose Broth. The results showed that evening primrose oil increased the growth of C. albicans and A. niger, while it suppressed the growth of S. aureus, S. epidermidis and P. aeruginosa even at the minimum concentration level (1mg/l). Borage oil was found to promote the growth of A. niger and S. epidermidis, but at concentration levels equal to or higher than 1mg/l it inhibited the growth of P. aeruginosa and C. albicans. However, at no concentration level, was it observed to leave any effect on S. aureus. It was also shown that some medical properties of evening primrose and borage could be attributed to antimicrobial effects of their oils.