Our study involved zircon U- Pb dating and in- situ Hf isotope, and whole- rock geochemical analyses for the medium- coarse- grained biotite granites associated with tin mineralization and the fine- grained hornblende biotite granites far away from the ore bodies from the Samoeng deposit, in Thailand. Zircon U- Pb ages show that the two granites were formed at 210.9±1.1 Ma and 206.5±1.0 Ma, respectively. Both granites are characteristic of rich in alkali with total alkali content of 5.81%~8.22%, relatively rich in Rb, Th, and Pb, weakly to strongly peraluminous (A/CNK=1.01~1.14) and low in TFeO/MgO (0.75~3.54) and 10000Al/Ga (2.21~2.66) values. The medium- coarse- grained biotite granites have primary muscovite and relatively high K2O/Na2O (1.56~2.50) and Rb/Sr (2.26~2.60) ratios, with high P content in their zircons, belonging to typical S- type granites. While the fine- grained amphibole biotite granites are wide development of amphibole, and have relatively low K2O/Na2O (0.45~1.11) and Rb/Sr (0.54~1.18) ratios, with low content of P in their zircons, which can be classified as typical I- type granites. The two granites have quite different Hf isotopic compositions. The medium- coarse- grained biotite granites have relatively low εHf(t) values (20.0 to 8.9), with the corresponding two- stage Hf model age of 1.8 to 2.5 Ga (average values of 2.0 Ga). In contrast, the fine hornblende biotites granite have relatively high εHf (t) values (4.6 to 5.5), with the two- stage Hf model age of 0.9 to 1.5 Ga (average values of 1.1 Ga). Both two granites were formed in a post- collisional tectonic transition (from compression to extension) related to the closed Paleo- Tethys Ocean, and have similar magmatic temperatures and moderate fractionated (DI=79.6~88.0, SiO2=67.57%~72.97%, and zircon Zr/Hf values=29.8~64.9). The mid- coarse- grained biotite granites were suggested to be derived from Paleoproterozoic metagreywackes with low magmatic oxygen fugacities (average ΔFMQ 4.93), whereas the fine- grained amphibolite biotite granites mainly originated from juvenile meta- igneous rocks with the input of ancient meta- sedimentary component, and have relatively high magmatic oxygen fugacities (average ΔFMQ 2.76). We think the nature of the source region and magmatic oxygen fugacity condition could be the most important controlling factors for tin mineralization in the Samoeng deposit.