Luteal Phase Defect (LPD) is responsible for 4% of infertility problems and can be presented by unexplained infertility. Unexplained infertility constitutes about 15% among infertile couples. In this study we tried to diagnose cases of LPD from patients presenting with unexplained infertility by color flow pulsed Doppler ultrasonography. Endometrial biopsy was done to the study group presenting with unexplained infertility dividing them into two subgroups; 1st group showing LPD and the other without LPD. Resistance Index (RI) was done to the study and control groups (normal fertile women). We found that the RI values were high in cases with LPD in comparison to normal values of the RI in patients without LPD and the control group.