Transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) were carried out to investigate the structural properties of the GaP/In-0.48(Al0.7Ga0.3)(0.52) P heterostructures grown on GaAs (0 0 1) substrates. The lattice-matched In-0.48(Al0.7Ga0.3)(0.52)P/GaAs material system could be used as a defect-free substrate because no lattice misfit exists between the In(AlGa)P layer and the GaAs substrate. Both TEM and HREM measurements indicated that there were not only misfit dislocations, but also microtwins at the GaP/ln(AlGa)P heterointerface. The mechanism of the microtwins formation is elucidated.