If the space of minima of the effective potential of a weakly coupled 2d quantum field theory is not connected, then a mass gap will be nonpertubatively generated. As examples, we consider two σ models compactified on a small circle with twisted boundary conditions. In the compactified CP(1) model the vacuum manifold consists of two points and the mass gap is nonperturbative. We studied the compactified SU(2) principal chiral model whose vacuum manifold is a single circle and calculated correspondingly the mass gap.