InthispaperwestudythetheoreticalpropertiesofmultigridalgorithmfordiscretizationofthePoissonequationin2Dusingamortarelementmethodundertheassumptionthatthetriangulationsoneverysubdomainareuniform.WeprovetheconvergenceoftheW-cyclewithasufficientlylargenumberofsmoothingsteps.ThevariableV-cyclemultigridpreconditionerarealsoavailable.