The microstructures and precipitation hardening of Ti1023 and Ti5553 alloys in the same solution treatment and step-quench aging treatment (ST-SQA) have been studied. The morphology of α phase about precipitation and distribution in different aging treatment were observed carefully by using SEM and TEM, and the variation of density and width of secondary α phase were analyzed. The hardness of these two alloys was measured. The result shows that: Ti1023 alloy is more likely to precipitate α phase than Ti5553 alloy due to low stability of ? phase. Ti1023 alloy precipitates α phase at 300℃ aging and get the maximum density of precipitation at 400℃,while Ti5553 alloy precipitates α phase at 450℃, and get the maximum density of precipitation at 550℃. As for Ti1023 alloy, the peak of hardness is obtained at 400℃ aging, while as for Ti5553 alloy, double peaks of hardness are shown at 350℃ and 550℃ aging, respectively. The variation of hardness was caused by the precipitation of secondary phase in ? phase for different aging temperatures. When aging temperature is below 400℃, the hardness of Ti1023 alloy depends on both α phase and ω phase, and the hardness of Ti15553 alloy depends on only ω phase. As aging temperature is over 400℃, the hardness of both Ti1023 and Ti5553 alloys depend on the density of precipitation and the size of α phase.