Abstract: In order to investigate mechanism of suspected linear defects indication in radiographic inspection of narrow gap welds in a nuclear power plant, microstructure and micro-hardness of the welds with suspected linear defects were studied by means of scanning electron microscopy and atomic probe. Causes of formation of micro-pores in microstructure were discussed, and distribution of micro-pores was characterized by porosity rate. The results showed that suspected linear defects were not true welding defects. Microstructure of the welds with suspected linear defects was austenite matrix and δ ferrite, and δ ferrite content was the same in different regions. Porosity rate of weld with suspected linear defects was about 0.06%, higher than that of the zone without suspected linear defects, 0.03%. Therefore, difference of porosity rate in microstructure was the main reason for suspected linear defects in radiographic inspection, and the difference did not affect microhardness. This study provided a new way to evaluate suspected linear defects in radiographic inspection.