Poly(para-phenylene vinylene) with crown ether substituents (C-PPV) could form nanoribbons through supramolecular assembly with K+ in dilute chloroform solution. The length of the nanoribbons increased with an increase in the standing time of the C-PPV/K+ solution. Experimental evidence to support the interaction between K+ and crown ether substituents was provided, and the growth mechanism of C-PPWK+ nanorods to nanoribbons was proposed. The influence of the length of the nanoribbons on the photophysics of C-PPV was also investigated.