Background: Oral health of pregnant women especially women who have Gestational Diabetes Mellitus (GDM) is very important, because of the adverse effects of pregnancy on the oral hard and soft tissues and vice versa and also because of its adverse effects on the fetus health.Objectives: The aim of this study was to evaluate the effect of education based on the theory of planned behavior (TPB) on oral health behaviors of pregnant women who have GDM in Birjand in 2018.Methods: This experimental study was carried out on 86 pregnant women, (43 pregnant women in each intervention and control group). Educational intervention based on TPB was performed in the intervention group. Two sessions of oral health education were conducted. Then, the questionnaire was given to the two groups. Data were collected and analyzed by SPSS 22 software at 0.05 significance level.Results: The mean age of participant in intervention group was 30.16±5.02 and in control group was 31.28±5.90.the results of the study indicating that after the educational intervention, the knowledge and attitude of the intervention group significantly increased compared to the baseline (P<0.001). However, intergroup comparative evaluation showed that constructions of subjective norms (intervention: 6.65±2.34, control: 7.56±2.18, P = 0.03), perceived behavioral control (intervention: 10.49±2.54, control: 10.76±2.21, P = 0.38), intention (intervention: 8.51±2.46, control: 13.7±2.37, P < 0.001) and performance (intervention: 10.77±2.11, control: 13.6±1.84, P < 0.001) did not increase under the influence of educational intervention in the intervention group.Conclusions: Considering that health education based on TPB in pregnant women with diabetes did not lead to changes in oral health behaviors; so it is suggested that oral hygiene instruction programs be included in the pre-pregnancy care schedule and periodic examinations be carried out to evaluate adherence and maintaining good oral hygiene among women.