Delayed hydride cracking (DHC) velocity was determined at 203, 227, 250 and 283^◦C using 17mm width curved compact toughness specimens machined from an unirradiated Zr–2.5 wt.%Nbpressure tube spool, gaseously charged with 60ppm of hydrogen by weight. Single CT specimen was used to determine DHC velocity at a constant temperature for a range of stress intensity factor (K_I) obtained by load drop method. For a given temperature and K_I >15MPam^1/2, DHC velocity was found to be practically independent of K_I. For 15 > K_I >10MPam^1/2, DHC velocity decreased significantly with decrease in stress intensity factor and extrapolation of the data suggested the threshold stress intensity factor to be about 9–11MPam^1/2 in the aforementioned temperature range. The activation energy associated with DHC was observed to be 35.1 kJ/mol.