The fluorescence of polyphenylene dendrimers and the intramolecular energy transfer in polyphenylene dendrimers containing a perylenediimide core have been investigated in this paper. Polyphenylene dendrimers composed of tens or hundreds of out-of-plane twisted phenyl units exhibit strong fluorescence, with quantum yields ranging from 0.2 to 0.5 depending on the dendrimer generation and its degree of functionality. The fluorescence of polyphenylene dendrimers can be efficiently quenched by the incorporated perylenediimide core, and consequently, a predominant emission from the core has been observed, indicating a very efficient intramolecular energy transfer.