Abstract Gestational diabetes mellitus (GDM) is associated with potentially harmful effects on the mother and fetus. Thus the identification of GDM offers a chance to improve pregnancy outcomes and identify women to target dietary & lifestyle health promotion. Aim of this research was to evaluate effect of health belief model based educational package on lifestyle among gestational diabetic women. Design: A quasi experimental design was utilized. Setting: This research was conducted in obstetrics & gynecological outpatient clinic and maternity hospital (high risk department) at the Zagazig University Hospital, Sharkia Governorate, Egypt. Sample: A purposive sample of (62) randomly women. Tools: Data were collected through three main tools: A self-administered questionnaire to assess women’s general characteristics and knowledge regarding gestational diabetes mellitus, health belief model scale, and health promoting lifestyle Profile II. Results: showed that there was a highly statistically significant difference regarding all knowledge items and total of all items of health belief model before and after GDM educational package implementation (P< 0.000). Moreover, there was a highly statistically significant difference in relation to all items of women’s lifestyle regarding GDM (nutrition, physical activity, stress management and health responsibility) before and after GDM educational package implementation (P < 0.001). Conclusion: There was a positive statistically significant correlation between total knowledge score, total lifestyle score and total health belief model score before and after GDM educational package implementation (p<0.001). Recommendations: Establishing strategies to enhance the women’s understanding of healthy lifestyle by applying this model to a large sample in various obstetrics and gynecological outpatient clinics.