We have grown two-layer InAs structures spaced by 6.5nm GaAs. Self-assembled InAs quantum dots (QDs) occur in the first layer with 2.5ML InAs coverage due to S-K growth mode. The second layer changes from 2D to 3D growth when the thickness of InAs is 1.5ML, which is obviously smaller than the critical thickness (1.7ML) of single InAs layer structures. Transmission electron microscopy shows that two-layer QDs with different heights have been obtained according to the mechanism of a vertically correlated arrangement. But only one photoluminescence peak related with large QD ensemble has been observed as a result of strong electronic coupling in InAs QD pairs.