A formulation of curcuminoid mixture having curcumin, demethoxycurcumin and bisdemethoxycurcumin and an essential oil of turmeric having 45% Ar-turmerone. The formulation gives enhanced bioavailability of bisdemethoxycurcumin, demethoxycurcumin and curcumin for up to 10 hrs. The curcuminoid mixture has curcumin, demethoxycurcumin and bisdemethoxycurcumin in a weight/weight ratio ranging from about 0.8:1:1 to about 1:1.1:1.2. A process of preparing a formulation of curcuminoid mixture having curcumin, demethoxycurcumin and bisdemethoxycurcumin in 0.8:1:1 to 1:1.1:1.2 weight/weight ratio and an essential oil of turmeric having 45% Ar-turmerone. A formulation for improved biological activity and bioavailability and tissue distribution of curcumin, demethoxycurcumin and bisdemethoxycurcumin.