We have investigated the effect of using different arsenic species on the degree of vertical self-alignment of InAs/GaNAs strain-compensated QDs in molecular beam epitaxy (MBE). We used the pairing ratio as a measure to evaluate the degree of vertical self-alignment. For the spacer layer thickness of > 20 nm, the QD pairing ratio for As(2) sample of 53% was smaller than that for As(4) of 62%. For As(2) sample, we think that the QD formation is more strongly affected by surface structure with a high step density than pairing effect resulting from strain field propagation. (C) 2010 Elsevier B.V. All rights reserved.