The age hardening behavior and the relevant strengthening mechanisms of Ti-10Mo-8V-1Fe-3.5Al (TB3) alloy have been investigated. A transmission electron microscope (TEM) was used to investigate the microstructures of aged and deformed TB3 specimens. It can be observed that dislocations can bypass the small α platelets in early aged samples. However, in late aged samples. the precipitated α platelets are large with high volume fraction, so that the β grains are divided into isolated β cells. In other words, the β grains are heavily refined. During plastic deformation, the dislocations generated in these β cells can not bypass the large α platelets by Orowan mechanism, but are confined within them. The strength model of the early aged TB3 alloy (aging time less than 2 h) is established by combining the law of mixtures with Orowan strengthening theory. For late aged TB3 alloy (aging time longer than 2 h), the strength model is established by combining the law of mixtures with fine grain strengthening theory. The predicated yield strength according to the modified strength models above can meet well with the measured values