[en] The strengthening behavior of beta titanium alloys at room temperature has been investigated under a wide variety of solution treated and aged conditions, and relationships between microstructures and mechanical properties will be discussed. Beta alloys can be strengthened mainly by the precipitation hardening mechanism, but increases in the strength of smooth, notched and precracked specimens are limited by ductility, notched tensile strength and fracture toughness, respectively. As a result, there is a critical value for strengthening for each specimen. Tensile ductility is strongly dependent on the beta grain size while fracture toughness is not. Therefore, the strength-ductility balance can be improved by grain refinement through applying the thermomechanical treatment while the strength-toughness balance cannot. Accordingly, there is a possibility of raising the critical strength when the application is limited by ductility, but it is very difficult to raise it when it is limited by toughness. The critical strength when limited by notched tensile strength is, at present, about 1,700 MPa in tensile strength, which is considered to be the maximum strength available for general applications