Objective To explore the effects of ICU diary on uncertainty in illness and hope among mothers of premature infants in Neonatal Intensive Care Unit (NICU). Methods A total of 80 premature infants in NICU and their mothers were divided into a control group and an intervention group according to the odd or even numbers of admission sequence. The control group received routine nursing, while NICU nurses additionally wrote ICU diary and sent to the mothers every day in the intervention group. The Perception of Uncertainty Scale (PPUS) and Herth Hope Index (HHI) were used to evaluate the mothers of the two group on the 2nd and 14th day in the NICU and the day of discharge from NICU. Results On the 14th day in the NICU and the day of discharge from NICU, the PPUS score of the intervention group was significantly lower than that of the control group, while its HHI score was significantly higher than that of the control group (P<0.05 for both). Conclusion Writing ICU diary and sending it to the mothers of premature infants in NICU, could alleviate the mothers′ uncertainty in illness resulting from parent-child separation, and improve their hope, which is conducive to transferring from hospital to family for the premature infants.