Objective:To investigate the agreement of ultrasound derived fat fraction (UDFF) with magnetic resonance imaging proton density fat fraction (MRI PDFF) on evaluating hepatic steatosis, and the performance of UDFF on diagnosing metabolic dysfunction associated steatotic liver disease (MASLD).Methods:One hundred and twenty-five volunteers and one hundred and seven inpatients who underwent abdominal ultrasound examination in Zhongda Hospital Southeast University from November 2023 to February 2024 were prospectively enrolled.UDFF and MRI PDFF were applied to evaluate hepatic steatosis. Spearman correlation test and Bland-Altman plot were applied to analyze the agreement of UDFF and MRI PDFF. Receiver operating characteristic curve (ROC) was applied to calculate the performance of UDFF on diagnosing MASLD.Results:In our participants, compared to individuals without hepatic steatosis, patients with MASLD had higher body mass index (BMI), waist-to-hip ratio, prevalence of diabetes mellitus, levels of alanine transaminase (ALT), aspertate aminotransferase (AST), alkaline phosphatase (ALP), γ-glutamyl transpeptidase (GGT), triglyceride, and UDFF (all P<0.05). The percentage of hepatic steatosis measured by UDFF and MRI PDFF was strongly correlated[ρ=0.873(95% CI=0.837-0.901), P<0.001]. UDFF performed excellent for diagnosing MASLD with an area under the curve (AUC) of 0.983(95% CI=0.956-0.995, P<0.001), and was better than semi-quantitative assessment based on two-dimensional ultrasound as well as ultrasound attenuation parameter. The optimal cut off value of UDFF to diagnose MASLD was ≥6%. Conclusions:The percentage of hepatic steatosis measured by UDFF and MRI PDFF agrees with each other, and UDFF obtains an excellent performance on diagnosing MASLD, so that UDFF should be considered a reliable imaging technique for quantitively evaluating hepatic steatosis and diagnosing MASLD.