In the analyzed materials of industrial incidents investigations, there is an incomplete volume of the established facts of the incidence and the causes of its occurrence. Often, when establishing the causes of incidents, biased decisions are made to establish the degree of guilt of the injured person in case of his gross negligence (from 5 to 100 %). The reason for this is the lack of regulatory and methodological documents regulating the procedures for establishing the fact of gross negligence and assessing the degree of guilt of the injured person as a percentage in an industrial incident. All this leads to an increase in the court proceedings, causing moral and material harm to the injured persons. The purpose of the work is to develop a methodology for assessing the degree of guilt of the injured person (insured) in case of his gross negligence based on of the equity principle of all the persons established by the commission for the investigation of an industrial incident in violation of the occupational safety requirements. The methodology is based on the methods of expert, point estimates and methods of incident analysis. Using this methodology, based on the materials of the investigation of a serious personal injury — the fall from the height that occurred with the driver of the tanker as a result of slipping on the slippery surface without the use of personal protective equipment, the main and related reasons of the incident were identified, the degree of guilt of the injured person and other participants of the event was established as a percentage. The developed methodology is aimed at the implementation of Article 229.2 of the Labor Code of the Russian Federation, makes it possible to bring clarity and transparency into the procedure for establishing the degree of guilt of the injured person (insured). The methodology will be useful for specialists in the field of occupational and industrial safety, state labor inspectors, insurers, representatives of the trade union organizations, i.e. all those who are directly involved in the commission for the investigation of industrial incidents.