Various age hardening models for wrought and casting Al alloys, based on semi-empirical equations consists of thermodynamics, precipitation kinetics and dislocation model, have been proposed to predict hardness and yield strength during and/or after aging treatments . Meanwhile recent developments of computational tool for precipitation simulation based on mean-field approach combined with CALPHAD-type thermodynamic and mobility database and physical properties calculated by first-principles calculations provide rigorous description of precipitation behavior, which in turn constitute sound age hardening model with minimal use of fitting parameters . The strength of Al-Si-Cu-Mg alloys after heat treatment has been reported to be much higher than for the ternary alloys, where different combinations of metastable precipitates have been observed depending on the alloy composition and heat treatment schedule. However, the precipitation sequence in Cu containing Al-Mg-Si alloys, thereby effects of alloying and aging condition on mechanical properties, has not been fully understood . In this presentation, effect of Cu addition on microstructure and mechanical properties of Al-Mg-Si alloy with composition of a typical A356 alloy is examined by simulation of precipitation sequence using integrated computer software(MatCalc, MatCalc Engineering GmbH), selected mechanical properties are predicted using a semi-empirical model, and importance of materials properties of stable and metastable precipitates are discussed.