The crack growth rates of un-irradiated and un-hydrided Zr-2.5Nb pressure tube material has been investigated in both air and in vacuum. Pre-cracked CCT samples made from pressure tube were tested in air atmosphere at 300 °C and 350 °C at different stress intensity factors. It was observed that till a particular stress intensity factor there is no crack growth and after which the crack growth rate increases with increase in stress intensity factor. For the test carried out in vacuum, it was observed that the crack growth rates for air is an order of magnitude higher than that of the crack growth obtained in the vacuum ambience.