A 10-T magnetic field was introduced to investigate precipitation behavior of Al-4wt.%Cu alloys during short time aging at 130°C. DSC, Vickers micro-hardness tests, EPMA and TEM were used to characterize aging process. The results show that high magnetic field improves the diffusion of Cu atom, lowers the stability of G.P.( I )zone and speeds up its solution, and lowers the precipitation temperature of the phases, thus accelerates the ageing course during the early stage at low temperature. Furthermore the high magnetic field has influence on the size of the θ″-phase. The micro-hardness of the field-treated specimen is always higher than the non-field ones, which indicates the high magnetic field strengthens the hardening effect.