Objective To evaluate the reliability of the Society for Fetal Urology (SFU) and Onen grading systems for fetal hydronephrosis in prenatal ultrasound according to the level of experience of the examiner. Methods We reviewed the prenatal ultrasound images of 146 fetuses (292 kidneys) that were diagnosed as having hydronephrosis between January 2005 and December 2014. One expert and two trainees assessed the prenatal renal ultrasound images using the SFU and Onen grading systems. The three examiners independently assessed each ultrasound image with both grading systems and reassessed the same images after 7 to 14 days. Cohen’s kappa statistic was used to estimate intra- and inter-observer reliability in prenatal ultrasound images according to training level. Results The intra-observer reliability of the SFU grading system (κ 0.873-0.945) showed almost perfect agreement and that of the Onen grading system (κ 0.749-0.913) showed substantial to almost perfect agreement. The overall inter-observer reliability of the SFU grading system (κ 0.620-0.825) showed substantial to almost perfect agreement and that of the Onen grading system (κ 0.618-0.724) showed substantial agreement. The weighted kappa value of inter-observer agreement was 0.223 to 0.400 for SFU grade 1 and 0.064 to 0.346 for SFU grade 3. For Onen grading, the interobserver agreement was 0.012 to 0.214 for grade 2 and 0.193 to 0.334 for grade 3. Conclusion Both the SFU and Onen grading systems showed good intra-observer agreement in prenatal ultrasonography. The inter-observer agreement was decreased in SFU grades 1 and 3 and Onen grades 2 and 3. Therefore, more focus should be given to SFU grades 1 and 3 and Onen grades 2 and 3 for trainees.