This paper reports the utilization of a high temperature superconducting (HTS) RF SQUID gradiometer in non-destructive testing (NDT). A two-hole HTS RF SQUID, capable of working in a noisy environment without any magnetic shielding, is employed as a gradiometer sensor without using flux transformer coils. This gradiometer, maintained at LN 2 temperature, is sensitive enough to detect small mock cracks in an aluminium alloy slab covered by another 25.4 mm thick solid aluminium alloy slab. Such an NDT device should have a significant impact on metal fatigue detection technology and demonstrates an important practical usage of high temperature superconductors.