We prepared a new conjugated polymer nanoparticle with the size of about 50 nm that is prepared by electrostatic assembly of cationic conjugated polymer PFO and anionic poly(l-glutamic acid) conjugated with anticancer drug doxorubicin (PFO/PG-Dox). The PFO exhibits good fluorescence quantum yield, photostability, and little cytotoxicity to meet the essential requests for cell imaging. In PFO/PG-Dox nanoparticles, the fluorescence of PFO is highly quenched by Dox by electron transfer mechanism, and thus the PFO is in the fluorescence "turn-off' state. After PFO/PG-Dox nanoparticles are exposed to carboxypeptidase or are taken up by cancer cells, the poly(l-glutamic acid) is hydrolysed to release the Dox, inducing the activation of PFO fluorescence to "turn-on" state. This multifunctional nanoparticle system can deliver Dox to targeted cancer cells and monitor the Dox release based on fluorescence "turn-on" signal of PFO, which concurrently images the cancer cells. The present work opens the door for new functional studies of conjugated polymer in simultaneous imaging and disease therapeutics.