In delayed hydride cracking (DHC) of Zr-2.5wt% Nb alloy, the hydride cluster at the crack tip has a critical length, which is a function of the stress intensity factor K-I and other parameters. When K-I > KIH, the threshold stress intensity factor, the hydride cluster must grow to this critical length before it will fracture. On the other hand, when K-I < KIH, there is a maximum length to which the hydride cluster can grow, and this length is insufficient for fracture i.e. less than the critical length. In this work, the lengths of the hydride cluster were experimentally studied for K-I < KIH and K-I > KIH near KIH. A modified experimental method was used, that permitted the hydride clusters to be formed and fractured individually. The hydride clusters were observed to be wedge-shaped, in agreement with the predictions by Metzger and Sauve (PVP, vol. 326, ASME, 1996). The lengths of hydride cluster measured in this work are compared with existing theoretical predictions. A good general agreement was obtained, but some differences are discussed.