Purpose: This study was conducted to identify the effects of a lifestyle intervention program on weight gain, dietary habits, fatigue and pregnancy stress, blood pressure, and neonatal birth weight, using Cox’s interaction model of client health behavior for overweight and obese women. Methods: This was a quasi-experimental research with a non-equivalent control group pre-post test design. A total of 52 patients who met the selection criteria, including 25 in the experimental group and 27 in the control group, were the subjects of the study; they comprised overweight and obese pregnant women who were receiving prenatal care at A and B women’s hospital in J province. The lifestyle intervention program ran for 12 weeks in total and consisted of interactions involving affective support, health information, and professional/technical competencies. The data collection period was from February 1, 2017 to August 31, 2017. Results: This study showed differences in the appropriate weight gain rate (c2=6.17, p =.013), suppression of an increase in fatigue (t=-2.32, p =.012), and an increase in pregnancy stress (t=-1.87, p =.034). Yet, no differences in physical activity, dietary habits change, blood pressure, and neonatal birth weight (p >.05) were found. Conclusion: The study findings indicate that this program could be an effective intervention for the control of appropriate weight gain, fatigue, and pregnancy stress. Therefore, a lifestyle intervention program based on Cox’s interaction model of client health behavior could be an efficient strategy for a positive health outcome of overweight and obesity pregnant women.