The effects of molybdenum (Mo) on the growth behavior of β-phase grain and hardness of a series of β forged Ti-Mo alloys under different solution treatment temperatures and solution treatment times were studied with an emphasis on the relationship between β-grain size and hardness. The average β-grain sizes under different solution treatment conditions were measured and analyzed．The results show that the β-grain size is not only relative to solution treatment temperatures and times but also relative to Mo contents from statistical results. The β-grain size does not always increase with increasing of solution treatment temperature, a limit value of the grain size will appear in certain temperature region and certain Mo contents. When the Ti-4Mo alloy is solution treated at the range of 900~1050 ℃, the activation energies for grain growth was 83.301 kJ/mol; and When the Ti-20Mo alloy is solution treated at the range of 750~900 ℃, the activation energies for grain growth was 272.16 kJ/mol. The growth behavior of β-phase grain is discussed in terms of grain growth thermodynamics and kinetics laws．Hardness results show that grain size has noticeable influence on hardness and hardness increases with decreasing of grain size.