The present study aimed to compare parenting stress, externalizing and internalizing problems of preschool children in working and non-working Mothers. This descriptive study is causal-comparative. In this study, all mothers of preschool children aged 4 to 6 years old in nurseries of Tehran City were considered as the statistical population. The sample size consisted of 230 subjects (115 working and 115 non-working), who were selected via multistage cluster sampling method. In order to collect the data, all participants were asked to complete Abidin's Parenting Stress Index (PSI, 1990) and Achenbach Child Behavior Checklist (CBCL, 1991). Iindependent t-test, multivariate analysis of variance and SPSS22 software were used to analyze the data. The results showed that both groups of working and non-working mothers showed a high level of parenting stress; however, there was no significant difference between parenting stress in the two groups (P <0. 05). Regarding the externalizing problems, there was a significant difference between the two groups. In terms of internalizing problems, although there was no significant difference between the total scores of two groups, the scores of two components i. e. emotional reactivity and sleep problems were significantly different (P <0. 05). Based on the results, both working and non-working mothers face parenting stress and children's problems. Accordingly, it is recommended to design and provide therapeutic and preventive interventions to improve maternal stress and children's problems.