Introduction. The mass introduction of new technologies led to the establishment of the institution of absolute liability in international law. At the same time, such activity is not prohibited by international law, which determines the legal nature of this type of liability. The aim of the article is to highlight the specifics of the principle of legal state activities in the establishment and development of absolute liability as a separate category of liability in international law. Results. It analyses the features of the absolute liability of the state caused by the ultrahazardous activities, such as such as the accident of causing damage and the absence of fault, the contractual nature of fixation, the primacy of norms, compensatory nature, etc. It is shown that such state activity is connected with the rapid development of scientific and technological progress and is not prohibited by international law. However, given that it is accompanied by increased danger, in the event of harm to another state, this activity, according to international law, is subject to material compensation. Сonclusions. Therefore, it is the legitimate activity of states, which is an ultrahazardous activity, determines the legal nature of absolute liability in international law, conditioning it as a separate type of state liability that requires special mechanisms for its consolidation and attraction to it. Key words: international law, ultrahazardous activities, absolute state liability,transbordary harm, acts not prohibited by international law, states liability.