Carbon nanodots (CDs) with high photothermal performance are ideal candidates for tumor photothermal therapy (PTT). Herein, we investigated the photothermal performance of CDs synthesized from urea and citric acid via solvothermal method in dimethyl sulfoxide (DMSO) at different temperatures. Photothermal conversion efficiency (PCE) of up to 61.3% was obtained in the CDs synthesized at 150 °C (150-CDs), which is much better than the CDs synthesized at 180 °C (180-CDs). By analyzing the morphologies, chemical structures, and absorption spectra of these CDs, we found that the photothermal effect of the CDs was due to the lattice vibration of their carbonized cores, in which moderate carbonization with high content of surface carbonyl group (C=O) contributed for enhanced absorption band in deep red region with improved photothermal performance. Moreover, we also demonstrated that 150-CDs with moderate carbonized cores exhibit much better tumor PTT performance in vivo than 180-CDs with higher carbonized cores. (Figure presented.)