The phase diagrams of triblock copolymers in cylindrical nanopores are investigated using the real-space self-consistent field theory in a two-dimensional space. We concentrate on pores with neutral surfaces and invariable diameters, whose rich variety of phases are considered to originate from pure geometric frustration. A series of triangular phase diagrams are constructed by varying the volume fractions for several sets of interaction parameters. These diagrams are sorted into three categories according to their interaction parameters. The confined phase diagrams exhibit several characteristic features that differ from those observed in the bulk. First, a rich variety of geometric frustration phases with strong symmetries, such as cylindrical and square, are observed in the triangular phase space because of the symmetry constraint in the geometric boundary. Second, the frustrated phases present some novel and complex features compared with those in the bulk, demonstrating that the confined morphologies much more sensitively depend on the subtle variation in the binary interaction parameters than those in the bulk. Meanwhile, the entropic energies of the symmetric melts with equal end block volume fractions are investigated to further understand the geometric frustration phase behaviors in the triangular phase diagrams. The reasonable formation mechanisms of the frustration phases are also discussed. (c) 2012 Elsevier Ltd. All rights reserved.