Si wafer direct bonding(SDB) technology is very attractive for both Si-on-insulator(SOI) electronic devices and MEMS applications. This paper presents on pre-bonding according to HF pre-treatment conditions in Si wafer direct bonding. The characteristics of bonded sample were measured under different bonding conditions of HF concentration and applied pressure. The bonding strength was evaluated by tensile strength method. A bond characteristic on the interface was analyzed by using FT-IR, and surface roughness according to HF concentration was analyzed by AFM. Si-F bonds on Si surface after HF pre-treatment are replaced by Si-OH during a DI water rinse. Consequently, hydrophobic wafer was bonded by hydrogen bonding of Si-OH$cdots$(HOH$cdots$HOH$cdots$HOH)$cdots$OH-Si. The pre-bonding strength depends on the HF pre-treatment condition before pre-bonding. (Min : $2.4kgf/cm^2{sim}$Max : $14.9kgf/cm^2$)