The effect of Mn as a surfactant on the formation of InAs nanostructures was studied. The shape of the nanostructures changed drastically when a layer of Mn was added on the growth front before InAs growth. When 6 ML2 (1) (1) (6 monolayers in (2 1 1) plane) of InAs was grown on GaAs (2 1 1)B with an initial Mn coverage of 0.5 ML, quantum wire-like structures (QWRs) were formed at growth temperatures ranging from 450 to 510 degrees C, With the same initial coverage of Mn, two kinds of nanostructures, quantum dashes (QDHs) and quantum dots (QDs) were formed at 400 or 425 degrees C, whereas only QDs resulted at 380 degrees C. Nanostructures on (1 0 0) and (3 1 1)B surfaces were also investigated and compared.