Polymer-conjugated nanoparticles are an important technology to control the stability, safety, and efficacy in drug delivery systems. Herein, we investigate self-organized mixed assemblies of a lipophilic drug candidate, curcumin (Cm), and a poly(oxyethylene) cholesteryl ether (PEG-Chol). Cm was assembled together with PEG-Chol to form nano-sized assemblies (around 10 nm) of assumed micelles. In contrast with the rapid decomposition of free Cm due to the hydrolysis, the Cm was highly stabilized in the nanoparticles, especially at below 40 mol% Cm. Cell viability assay revealed that the cytotoxic activity of the Cm/PEG-Chol nanoparticles against myeloma cells is higher than those of free Cm in a comparison at 1 microM. On the other hand, both the Cm/PEG-Chol nanoparticles and PEG-Chol micelles had significant cytotoxicity to the myeloma cells at 5 microM. Taken together, the present Cm/PEG-Chol system offers a stable nanoparticle encapsulating Cm which can be injected as a liquid. Cm and vehicle micelles will damage the cancer cells cooperatively.