In this study, two isolates of marine yeast TH1 and TH4 from sea water samples collected from industrial zone in Nghi Son, Thanh Hoa were demonstrated for biofilm formation and highly diesel oil degradation. The yeast strains TH1 and TH4 were then identified by ITS1-5.8rRNA-ITS2 sequencing analysis and showed homological coefficient of 99 percent and 97 percent with Candida viswanathii and Candida tropicalis, respectively. The influences of initial pH, NaCl concentrations and temperature on biofilm formation were investigated. As a result, the optimal biofilm formation conditions of TH1 were pH of 5.0,6.0 and 7.0, NaCl of 1.5 percent and at 30°C
while TH4 were pH of 3.0 upto 7.0, NaCl 0.5 - 1.5 percent and at 30°C. By using 2 percent of diesel as sole carbon source, the oil degradation capacities of the two strains with planktonic and biofilm types were examined. At biofilm type, the TH1 and TH4 degraded 83 and 63 percent of total diesel oil, respectively
meanwhile at planktonic type both strains degraded oil at the same yields of 70 percent. Using biofilm formed by mixture of two strains to estimate degradation of 2 percent diesel oil, all n-alkanes from C8 to C20 were totally degraded after 7 days by gas chromatography analysis. These results showed potential application of marine biofilm forming yeasts in treatment of petroleum oil polluted in water in Vietnam.