Self-assembly of cylinder-forming diblock copolymers confined in cylindrical nanopores is studied systematically using a simulated annealing method. The diblock copolymers form hexagonally packed cylinders in the bulk with a period L0, whereas novel structures spontaneously form when the copolymers are confined inside cylindrical pores. It is discovered that the sequence of structures is controlled by the ratio between the pore diameter (D) and L0, as well as the selectivity of the pores. For selective small pores (D/L0 2.7), the outer ring of the minority block-domain forms helices or stacked toroids, while the inner structure repeat the sequence of structures observed in smaller pores. ...