Nurses are among the working groups who would be affected by themental illnesses and the stresses within their environment, thus depression and anxiety are their common problems. The aim of this study is to compare depression, anxiety and stress among the nurses in critical care and internal-surgical unitsat the selected hospitals of social security organization of Tehran. This descriptive-comparative study is performed on 220 nurses working in the selected hospitals of Tehran social security who are selected by simple random sampling in the period of March 2016 to October 2016. The necessary data are collected using a two-part questionnaire, including demographic information and a DASS-42 standardized questionnaire. This 42-item questionnaireis formed from three subscales of 14 questions related to depression, anxiety and stress. Data are analyzed using spss-22 software. The working place of nurses are significantly associated with incidence of depression and stress among the nurses in the critical care unit (sig<0.002). But incidence of depression and stress among the nurses in internal and surgical units is less than the critical care units. The anxiety scale has no significant difference between the two groups sig: 0.868. Depression is mostly related to hemodialysis and stress is mostly related to CCU. No significant correlation can be seen that would be associated with incidence of depression, anxiety and stress between variables of age, type of employment, education, marital status and working experience (p> 0.005) But there is a significant correlation between gender and incidence of depression, anxiety and stress among the nurses (p<0.005) and the incidence of mood disorders in nurses and the rate of depression, anxiety and stress in female nurses is more than male nurses (p <0.005) High levels of depression and stress among thenurses in the critical care unitis alarming. Progress in providing service and adequate supports leads to the health of nurses and therefore to the development of health in the society.