Uranium monocarbide (UC) was successfully synthesized by the Pechini-type in situ polymerizable complex technique (IPC) with the organic matter as the only carbon source. In the aqueous process, a mixture of citric acid (CA) and mannitol with UO22+ was polymerized to form a spongy-like organic polymeric precursor without any precipitations. The structural evolution and formation mechanism of the precursor were investigated using XRD, DSC-TG, SEM (EDX), TEM, and FT-IR. XRD results demonstrated that UC was obtained with the UO22+/mannitol/CA molar ratios of 1.0/0.3/1.0 at a low temperature of 1400 degrees C. SEM and TEM analyses revealed that the UO2 nanoparticles were uniformly distributed in the carbon matrix to form UO2/C nanocomposites, and submicrometer-sized ellipsoidal UC particles cemented together. FT-IR showed that a UO22+-CA chelated structure was firstly obtained, achieving the molecular scale mixing of uranium and C. Then the in situ charring guaranteed the intimate contact of UO2 and C, leading to a low reaction temperature in carbothermal reduction owing to a short diffusion distance.