Coherent InAsxP1-x islands were formed on the InP (1 0 0) and the InP (3 1 1)B surfaces by As/P exchange reaction under As flux and atomic H irradiation at 480 degreesC. Islands on the InP (1 0 0) surface were found only formed at the step edges with an extremely low density. Contrary to the InP (1 0 0) surface, dense islands were formed on the InP (3 1 1)B surface. Especially, the InAsxP1-x island arrays on the InP (3 1 1)B surface exhibited a significant narrowing of the size dispersion and a surprisingly strong nearest-neighbor correlation. It is suggested that long-range elastic interaction between self-assembled islands via the anisotropically strained substrate play a crucial role in this self-organization process.