This contribution analyses influence of thermo mechanical conditions of magnesium alloy - type AZ31 forming on its flow stress. Temperature and strain/deformation rate belong to the essential thermo mechanical parameters of strain/deformation process. A testing cylinder made of selected Mg alloy was strained using pressure at warm temperatures within defined two degrees of nominal strain 30% and 60% and strain rate of 5 s-1. The pressure test at warm temperatures was conducted at testing temperatures 250, 300 and 350°C. It is important to explore the temperature influence on AZ31 alloy flow stress in order to reduce energy consumption of formed pieces production. Surface quality and precision of required dimension will improve, as well. The experiment was aimed at graphic evaluation of temperature influence on flow stress of Mg alloy AZ31B. Resulting form flow stress curve it is possible to read out its value for particular strain. These values are essential for calculation of forming force and work. For magnesium alloy AZ31 warm forming at temperature interval of 230 - 425°C is typical. The pressure tests were realized within the temperature interval of 250 - 350°C, i. e. at temperatures belonging to lower limit of recommended temperature interval, with the aim of acquirement as much information as possible on Mg alloy behavior at low forming temperatures.