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–2.5wt.% Nb pressure 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 35.1kJ/mol.