We study a quantum frustrated Heisenberg model on the two-sublattice bcc crystal taking into account nearest-neighbor and next-nearest-neighbor interactions on one sublattice, J1 and J2 respectively. The classical ground-state spin configuration shows a noncollinear phase. The method used is an improved self-consistent Green function method: we calculate the free energy F of this system and determine the angle at each temperature (T) at the minimum of F, instead of the minimum of the internal energy as previously done. The properties at finite T are studied for collinear and noncollinear phases. The phase diagram in the space (J2/J1,T) is shown. We confirm here by our analytical method the existence of the partial disorder in a frustrated vector spin system discovered recently by MC simulation. The present method opens an efficient way to study noncollinear spin systems over the whole range of temperature and might help to discover new phenomena in more complicated spin systems. © 1998 American Institute of Physics.