Microstructure and mechanical properties are investigated for 0.15C-1.5Mn-1.5Al TRIP aided cold rolled steels containing $0.05wt\%P$ and $0.1wt\%P$. Despite of the complete replacement of Si by Al, the TRIP steel shows tensile strength of 700MPa and total elongation of $35\%$ by addition of $0.1wt\%$ P. Tensile strength of P added TRIP steels is not only affected by the solid solution hardening but also the volume fraction of retained austenite. As P content increases from $0.05wt\%$ to $0.1wt\%$, tensile strength and volume fraction of retained austenite are increased, but elongation is decreased. The lower stability of austenite in $0.1wt\%$ P added steel is responsible for the decrease of the elongation.