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Alteration in the subaqueous basaltic rock of the Lam Narai volcanic belts, Lop Buri province, central Thailand has resulted in the formation of bentonite deposits. The bentonite deposits have been geological, mineralogical and geochemical studied to determine their genesis. The characteristics of the bentonites were examined by using X-ray Diffractometer and Scanning Electron Microscope with Energy Dispersive Spectroscopy. The mineralization zone of bentonite shown X-ray diffraction patterns of montmorillonite as the major constituent with minor hematite and calcite. Morphology of the minerals and groundmass in the bentonite deposits display the "cornflake," "oak leaf," or "cellular" textures that are typical and representative of montmorillonite in smectite group and composed of Si, Al, O, Ca, Mg and Fe in their chemical compositions. Geochemistry of the bentonite and their parental volcanic associations were analyses by using X-ray Fluorescence Spectrometry for major and trace elements. The representative samples were selected for the analysis of the rare-earth elements by using Inductively Coupled Plasma-Mass Spectrometry. The AI-CCPI alteration box plot revealed that the bentonite is mainly altered by hydrothermal alteration associated with basaltic layer in the Lam Narai volcanic belt. The basaltic layers in this area are composed of pillow lavas, inter-pillow and basic glassy inter-pillow (hyaloclastite) that indicated the subaqueous environment. The smectite formed by hydrothermal alteration from altered basic glassy inter-pillow (hyaloclastite) in the basaltic layers. Hydrothermal fluids in this area are possibly derived from the major fault along Pa Sak River or the youngest intrusive rocks in the Lam Narai volcanic terrain.