The problem is solved about simple – supported beam from shape memory alloys with taken into account rheonomous properties of these materials in framework of analog of viscoplastic model. The beam is loaded in the regime of martensite inelasticity. It is stated that this problem can be considered in framework of Lyapunov stability criterion. The solution is found in framework of conception occurring everywhere structural transformation. The critical load can be found using Euler formula with changing the Young’s modules to tangent modules be calculated in loss stability point using martensite inelasticity diagram correspondent to ultimate slow processes. The jump-like changing of critical stresses with beam length decreasing is investigated.