Mg-20.5 at% Sc alloy with hcp (α)+bcc (β) two-phase was investigated to understand the effects of aging treatment at 473 K on microstructure, hardness and tensile properties. A Mg-Sc alloy ingot was prepared by induction melting in an Ar atmosphere, and then hot rolled at 873 K followed by cold rolling into a sheet. The rolled specimens were annealed at 873 K to obtain an α+β two-phase microstructure. The annealed specimens were then aged at 473 K for various time. Vickers hardness of the α+β two-phase alloy drastically increased after a certain incubation time and then reached a maximum hardness of 142.8 HV. The incubation time of the Mg-20.5 at% Sc alloy with the α+β two-phase was longer than that of the same alloy with a β single-phase. Ultimate tensile strength (UTS) and elongation of the as-annealed specimen were 280 MPa and 28.2%, respectively. Meanwhile, the specimen aged at 473 K for 14.4 ks showed a UTS of 357 MPa and an elongation of over 12%. The specimen aged for 18 ks showed a higher UTS of 465 MPa, while keeping a better elongation of 6.9%. The age hardening of the Mg-Sc alloys was found to be due to the precipitation of very fine α phase in β phase.