Introduction: The neonatal intensive care unit (NICU) is a nursing care-intensive setting where the patient-to-nurse staffing ratios affects the quality of care and would be accompained with outcomes such as readmission neonates and nurses' fatigue or job satisfaction. Aim: This study aimed to determine patient-to-nurse staffing ratios and its consequences for premature infants and nurses. Methods and Materials: This descriptive correlational study was done with a sample of (80 nurses and 245) preterm infants in the neonatal intensive care units in three selected hospitals Tehran (2014). Study outcomes for nurses were fatigue and job satisfaction which examined by "fatigue scale nurses'' (Rassouly & et., al 2012) and "job descriptions index"(Smith & et., al 1969). They had content validity and their reliabilities were &alpha=0.92. The outcome of infants was readmission within one month after discharge, which followed by phone calls to parents. Data were analyzed by 2, t-test, Kroscal-Wallis, Mann-Whiteny, linear and logistic regression by using SPSS1705/0 =&alpha)). Findings: In total, the mean patient-to-nurse staffing ratios in hospitals was (0.28  0.018) in other words, a nurse was for more than 4 infants. There were no correlation between patient-to-nurse ratios, premature infants` readmission (P=0.051) and nurses` fatigue (P=0.055) in regression analysis significantly. But job satisfaction was significantly correlated with patient-to-nurse ratios so that for one point increase in the patient-to-nurse ratios, 9.58% increased score of job satisfaction (P=0.001). Conclusion: The patient-to-nurse staffing ratios as one of the factors affecting on nurses' job satisfaction, so, nurse managers would be used it for increasing NICU nurses' satisfaction.