The precise location of unlar osteotomy remains a subject of ongoing controversy in chronic Monteggia fracture (CMF). The purpose of this study was to analyze the influence of different levels of ulnar osteotomy on redislocation in CMF. We retrospectively reviewed 18 children following our previous series. Except the baseline characteristics and radiographic parameters such as ulnar osteotomy angle, maximum interosseous distance (MID) and proportional ulnar length (PUL), we introduced a new parameter: proportional ulnar osteotomy (PUO) which represents the quantitative level of proximal ulnar osteotomy. Based on the value of PUO, we divided it into two intervals: appropriate PUO range (1/5< PUO <1/3) and inappropriate PUO range (PUO >1/3; PUO <1/5). The relationship between these indexes and redislocation was analyzed. According to the reduction state of radial head, patients were divided in two groups: reduced (15/18) and redislocation (3/18). Only PUO range differed significantly (P=0.043) between the two groups, with a notably higher number of patients showed an osteotomy between 1/5 and 1/3 of ulna in reduced group. Combining PUO range with radiographic parameters (osteotomy angle and post-PUL) improved the accuracy and specificity over using osteotomy angle and post-PUL (accuracy, 94.44% vs. 83.33%) (specificity, 93.33% vs. 86.67%, P=0.008). This combination further enhanced the predictive capability for detecting the risk of redislocation in CMFs. Ulnar osteotomy between the proximal 1/3 and 1/5 appears to provide a much safer and more stable radiocapitellar joint in CMF.