Basic principles of nonlinear thermodynamics are used to develop a theory of tubulin assembly. The presence of GTP/GDP switch provides different ways for tubulin aggregation. The GTP-tubulin assembly results in determining the regime of limit cycle favoring growth of tubulin spiral with the formation of microtubules. The GDP-tubulin assembly, on the contrary, results in forming double rings. The difference between the GTP- and GDP-tubulin assemblies is that the former is a dynamical oscillation process while the latter is no more than a transition from a weakly nonequilibrium point to the equilibrium state along a thermodynamical path. Corresponding equations for the GTP- and GDP-tubulin assemblies are proposed.