Background and Objective: Previous studies indicated that morphine consumption during pregnancy could inhibit embryos development. Present study further evaluated the effects of oral morphine consumption on the placenta lacunas development in ten day pregnant Wistar rats. Material and Methods: Female Wistar rats (W: 170-200 gr) were used in the present study. Experimental group were received morphine (0.05 mg/ml of tap water) after one night coupling with male rats for mating. On the day 10th of pregnancy, the pregnant animals were killed with chloroform and the placentas and uterus were removed surgically and fixed in 10% formalin for twenty days. The fixed placentas were processed and stained by H & E method and evaluated for their development. Thickness of layers, surface area of lacuna, as well as the number of cells in both maternal and fetal parts of the placentas was assessed by light microscopy. Results: Our results indicated that the layer thickness of fetal portion and surface area of lacuna of the fetal and maternal portion of placenta reduced in experimental group. In addition, maternal portion layer thickness and cell number of the fetal and maternal portion of placenta increased in the experimental group. Conclusion: Our results showed that oral morphine consumption could inhibit natural function of placenta lacuna and fetal cell development.