Passive gamma scanning (PGS) for detecting the variations in relative PuO2 content (composition) in (U,Pu) mixed oxide (MOX) fuel pins with low PuO2 content (0.4%) for pressurized heavy water reactors is demonstrated. Experiments are carried out with MOX pins of varying compositions with a NaI(Tl) detector. An annular detector was used to improve the counting statistics, and it was demonstrated that a change of PuO2 content of ±0.02% could be detected at a very low level of PuO2 content of 0.4%. It was also observed that a larger-sized PuO2 agglomerate of 1 mm located near the surface of the pellets could be detected during PGS of the welded fuel pins.