The mass of substructure is a main component that effects on the period and subsequently forces and displacements of the isolated bridges. But some bridge design codes such as AASHTO guide specification, ignores it in simplified method of analysis. The substructure mass consists of four components: pier mass, cap beam mass, pier rotational moments of inertia, and cap beam rotational moments of inertia. This paper investigates the effects of mentioned components on the longitudinal period of base isolated bridges. For this purpose, the analytical models of isolated bridge and calculation of period are presented in longitudinal direction. The periods obtained from analytical model were compared with those obtained from finite element model. The results shows that the substructure rotational moments of inertia doesn't have any impact on the longitudinal period; but the cap beam mass have a significant impact on the longitudinal period of base isolated bridges with tall piers. The ignorance of cap beam mass causes 8% error in the longitudinal period calculation of tall isolated bridges. The results also shows that the simplified method of AASHTO guide specification isn't applicable in some bridges and may causes 15% error. By increasing the ratio of isolator stiffness to substructure stiffness, the error of period calculated by AASHTO model, increases.