Nanoparticles capable of accumulating in tumor tissues are promising materials for tumor imaging and therapy. In this study, two radical nanoparticles (RNPs), denoted as 1 and 2, composed of self-assembled ureabenzene derivatives possessing one or two amphiphilic side chains were demonstrated to be candidates for metal-free functional magnetic resonance imaging (MRI) contrast agents (CAs). Because of the self-assembly behavior of 1 and 2 in a saline solution, spherical RNPs of sizes similar to 50-90 and similar to 30-100 nm were detected. In a highly concentrated solution, RNP 1 showed considerably small water-proton relaxivity values (r(1) and r(2)), whereas RNP 2 showed an r(1) value that was around 5 times larger than that of RNP 1. These distinct r(1) values might be caused by differences in the self-assembly behavior by a hydration or dehydration process. In vivo studies with RNP 2 demonstrated a slightly enhanced T-1-weightedimage in mice, suggesting that the RNPs can potentially be used as metal-free functional MRI CAs for T-1-weighted imaging.