The directional energy flow and intense light emission of the nanostar dendrimer (C460H424) that consists of phenylacetylene oligomers and a perylene molecule are studied from the viewpoint of molecular orbital interactions. The method of choice in the present study is the density functional theory and the simple Huckel molecular orbital (MO) method. Although the total density is homogeneous in this pi-conjugated system, the amplitude of an individual orbital is localized well in a small fragment, due to the m-phenylene joints involved. The highest occupied MO (HOMO) and lowest unoccupied MO (LUMO) are localized well in the perylene core: the HOMO-2, HOMO-1, LUMO+1, and LUMO+2 are localized in the first-generation branches; the HOMO-3 to HOMO-6 and LUMO+3 to LUMO+6 are localized in the second-generation branches; and the HOMO-7 and below and the LUMO+7 and above are localized in the third- and fourth-generation branches. These localized pi orbitals play an essential role in the directional, multistep energy flow from the periphery to the center of the nanostar dendrimer. To investigate important photoexcited states of the nanostar dendrimer, singlet excitations of a linear oligomer involved in the dendrimer are calculated with the time-dependent density functional theory. The fragment molecular orbital method is applied to analyze how the orbitals of the nanostar dendrimer are localized in space as well as in energy. A method of reasonable partition of the nanostar dendrimer is theoretically considered in terms of interactions between fragment molecular orbitals.