Built-up edge (BUE) is an accumulation of the workpiece material against the rake face of cutting tools. The existence of BUE can caused tool wear and also led to bad surface finishing on the workpiece during the milling process, whereas some part of the workpiece is removed mechanically. Formation of BUE requires the rework on the workpiece. It is unfavorable to manufacturing industry as it will consume more cost and time. Therefore, the determination of critical factor of BUE formation during stainless steel milling are important to be investigated. This research aimed to determine the critical factor of BUE formation in stainless steel milling. Initially, several journal and articles were reviewed to find the possible causes that lead to BUE formation in stainless steel milling process and content analysis was performed to list down all possible causes. Next, ten individualists that expert in stainless steel milling are also involved in data collection. After the data collection, Shapiro-Wilk analysis (normality test) and analysis of variance (ANOVA) test were performed to analyze the data and determine the critical factor that lead to BUE formation in stainless steel milling. Lastly, the data will be examined and conclusion was being finalized. As a result, there are six identified critical factor of BUE formation in stainless steel milling. The critical factors are cutting speed, tool rake angle, cutting temperature, microcrack formation, feed rate and tool rake temperature. The finding of this research can contribute to any party that interest in this field to develop new technology or system that can completely prevent BUE formation during stainless steel milling process, thus avoid rework which are cost and time consuming.