Background: Neonatal intensive care unit (NICU) developmental care refers to interventions in the infant's environment in neonatal intensive care unit to reduce environmental stresses and provide greater family-infant compatibility. Objectives: The aim of this study was to assess current developmental care performance and factors influencing implementation of this care at NICUs in Tehran, Iran. Methods: This cross-sectional study as part of a larger mixed method study assessed the quality of developmental care in Tehran NICUs. The study was performed on 400 nurses working in level III NICUs in 27 hospitals of Tehran during years 2014 and 2015. Data were collected using two scales, "NICU developmental care assessment (NDCA) scale " and "NICU developmental care structural (NDCS) checklist". The scales validity was detected by content, face and construct and reliability by internal consistency and testretest. The overall and each domain score were calculated in percentages. Data were analyzed using the SPSS-21 software using descriptive statistics tests and regression analysis. Results: The total score of quality of providing developmental care was 74.84%. Among all domains, "daily routine care" had the highest score (85.67%) and "sleep and pain care" had the lowest score (66.63%). Total score of structure was 43.06%. The number of neonates admitted per day (B -0.328, P = 0.019) and number of infants managed by each nurse (B -2.543, P = 0.019) were significant predictor variables for better quality of total developmental care. Conclusions: The number of infants cared and admitted were the only significant factors contributing to high quality of developmental care. Therefore, it is reasonable to assume that in case of Tehran, work-load and structure of services should receive greater attention.