Abstract: A crack with angle less than 30° to the weld axis tested by yoke magnetic powder testing was found on a C-Class weld of a boiler, which was difficult to be detected in theory. Yoke magnetic powder testing was conducted, such as magnetic field indicating lines testing and field indicating lines by yoke suspending testing. Testing results showed that when testing by yoke at a limited space, if the magnetic field was sufficiently strong, the testing could still be proceeded for the defect at the magnetic yoke surrounding, but more attentions were needed to be paid to distinguish the relevant and non-relevant indications.