This study aims to investigate the impact of a health belief model (HBM)-based educational intervention on knowledge, beliefs, self-reported practices, gestational and postpartum weight in women with gestational diabetes mellitus (GDM). Subjects and Methods: A cluster randomized controlled trial was performed, with randomization at the level of Primary Health Care centers in three Egyptian cities. Eligible women with GDM were enrolled at 24 weeks pregnancy. The intervention group (n = 103) received health education intervention based on the HBM construct. Control subjects (n = 98) received the usual care. The outcomes measured were: women's knowledge, beliefs, self-reported practices, gestational weight gain (GWG), and postpartum weight retention. Patients were investigated at baseline, at end of pregnancy, and at 6 weeks postpartum. Results: After the intervention, percentages of women who had high knowledge and beliefs scores had significantly increased from less than 50 % to more than 70 % in the intervention group (p < 0.001). More women in the intervention group reported practicing exclusive breast feeding (85.4 %) and screening for T2DM (43.7 %) at 6 weeks postpartum compared to the control group (63.3 and 19.4 % respectively) (p < 0.001). More women with excessive body mass index in the intervention group (65 %) compared to the control group (11.6 %) were meeting recommended GWG (p < 0.001), and postpartum weight (37.7, and 20.3 % respectively) (p < 0.01). Conclusion: This intervention significantly improved knowledge, beliefs, self-reported practices, and gestational and postpartum weight in patients with GDM. Further research is needed for investigating the effectiveness of applying early, multi-phase, and longer intervention.