The theory of the Mössbauer isomer shift is developed from the pseudopotential and Born-Oppenheimer approximations. The result contains four parts, the free-electron contribution, the. crystal potential with the Debye-Wailer factor, and both intraband and interband self-energy corrections. The formulism provides us a base to study the isomer shift in β-Sn at the finite temyerature in the subsequent papers of the series. The special case for T=O is also discusseed and compared with Inglesfield result.