With the development of Low Earth Orbit (LEO) satellites, differential code biases (DCBs) estimation based on onboard observations has been widely studied. In this contribution, a method of BDS and GPS DCBs estimation based on onboard observations is present. BDS and GPS onboard observations from Chinese Fengyun- 3D (FY-3D) and Fengyun-3C (FY-3C) are used to estimate BDS C2I-C7I and GPS C1C-C2W DCBs. Firstly, DCB estimation results based on FY-3D onboard observations are analyzed. When jointly processing BDS+GPS onboard observations, the stability of satellite and receiver DCBs for both BDS and GPS has a better consistency with the DCB products of DLR and CAS than that for the single-system solutions (BDS-only solution and GPS-only solution). This is reasonable because more onboard observations are used in BDS+GPS solution, which can improve the solution strength of DCB estimation. Compared with the FY-3C solution, the FY-3D solution can achieve a more stable satellite DCB with a stability improvement of 30%, 51%, 61% and 47% for GPS, BDS GEO, IGSO, and MEO satellites respectively. Meanwhile, the receiver DCB of FY-3D is more stable than that of FY-3C as well. These improvements of satellite and receiver DCBs can be owing to the enhancement of FY-3D GNOS instrument, which can provide more observations with higher quality. Furthermore, both FY-3D and FY-3C onboard observations are processed together to estimate BDS and GPS DCBs. Compared with FY-3D solution, the stability of satellite DCB can be improved by 19%, 1% and 5% for GPS, BDS IGSO and MEO satellites DCB respectively, when both FY-3D and FY-3C onboard observations are jointly processed.