Objective　To explore the effects of endoplasmic reticulum (ER) stress in the hyperoxia-induced lung injury in premature rats. Methods Forty-eight premature Wistar rats were randomized into two groups 12 hours after birth: hyperoxia group (n=24) inhaled 95% oxygen and control group (n=24) inhaled air. Eight rats were sacrificed in each group on day 1, 3, 7 after the treatment and the left lungs were embedded. The pathological changes in the HE stained sections of lung tissues were observed. The expressions of ER related protein ERp57 and c/EBP homologous protein CHOP were detected by immuno histochemistry and the apoptosis of lung cells was detected by TUNEL analysis. Results The typical pathological characteristics of acute lung injury were observed in hyperoxia group. The expressions of ERp57 and CHOP were increased with the exposure time in hyperoxia group, and were significantly higher than in control group (P < 0.05). The apoptosis rate of lung cells in hyperoxia group was significantly higher than in control group (P < 0.01). There was significant positive correlation between cell apoptosis index and expressions of Erp57 and c/EBP homogeneous protein. Conclusions ER stress initiated apoptosis participates and plays an important role in the process of hyperoxia-induced lung injury in premature rats.