Introduction: Rapid decline of the total fertility rate in Iran in recent decades has caused serious demographic challenges to the country. One important reason for this decrease is single-child behavior adopted by Iranian families. Objective: The present study was conducted to assess the effectiveness of education based on the Theory of Planned Behavior (TPB) on childbearing intention in single-child women. Materials and Methods: This randomized field trial with the pretest-posttest design was done on 72 single-child women presenting to health centers in Mashhad City, Iran, in the intervention (37 persons) and control (35 persons) groups from August 2019 to February 2020. Research tools included a demographic and obstetric questionnaire and a researcher-made childbearing intention questionnaire based on TPB. The intervention group received three 110-min sessions of education, which was based on TPB. The control group received routine health center care. The study data were analyzed by using the Kolmogorov-Smirnov, Shapiro-Wilk, Independent t-test, Mann-Whitney U test, repeated measures Analysis of Variance (ANOVA), Freidman test, post hoc tests of Bonferroni and Dunn, the Spearman rank correlation coefficient, and the Generalized Estimating Equation (GEE) model. Results: The mean±SD age of the participants were 32.6±4.7 and 32.9±4.7 years in the intervention and control groups, respectively, and most of the women had university-level education in both groups. Changes in childbearing intention from the pre-intervention stage were significantly higher in the intervention group than the control group immediately (P=0.001) and then one month after the intervention (P=0.001). The results of the GEE model showed that the childbearing intention was significantly higher in the intervention group (β=0.68, 95% CI; 0.449-0.911, P=0.001,), and there was a significant positive relationship between attitude and childbearing (β=0.023, 95% CI; 0.013-0.033, P=0.001). Conclusion: Education based on the theory of planned behavior increased childbearing intention in single-child women in this study, so this educational program is advised for promoting childbearing.