Abstract: A direct competitive enzyme- linked immunoassay ( dc ELISA) was developed for determination of sildenafil. The results showed that the optimized parameters were as follows :100ng/m L for coating antigen concentration, 70 times dilution of enzyme conjugated antibody, dilution solution of sildenafil in 0.01mol/L PBST (p H =9) and reaction time for 50 min. This developed method presented an IC50 of 1.47ng/m L, a linear range (IC20~IC80) of 0.12 ~17.65ng/m L, a detection limit IC15 of 0.0845ng/m L. Intra-and inter-batch relative standard deviations (RSD) were below 20%. The recoveries of sildenafil in spiked samples ranged from 86.0%~111.0%.In conclusion, the dc ELISA was a practical method for trace detection of sildenafil in real samples.