Carbon based dots (CDs) have attracted broad attention exhibit due to the unique optical properties. However, the exact origins of their optical properties are still controversial. Citric acid (CA) coupled with some amino group-containing small molecules are believed to be ideal precursors for the synthesis of high luminescent CDs through various thermal treatment processes. Herein, CA coupled with four amino group-containing small molecules are chosen as models to synthesize CDs for a systematical study on the photoluminesce (PL) properties. It is found that the PL properties of CDs are resulted from the synergistic effect of the contained luminescent pyridine-derivatives and the defect states. A reasonable mechanism of PL emission from the CDs has been proposed. The results presented here must be critical for understanding the origins of PL, and also for preparing CDs with strong and wavelength tunable PL emission. Display omitted