Cross-section transmission electron microscopy and high resolution electron microscopy were done to investigate the microtwins in GaP/(AlxGa1-x)(0.52)In0.48P/GaAs double heterostructure light-emitting diodes. There is a difference between the microtwins extending from the GaP/ (AlxGa1-x)(0.52)In0.48P interface into the GaP layer, and the microtwins existing in the GaP layer. High resolution electron microscopy measurements show that the length of microtwins extending from the GaP/AlGaInP interface along (112) directions is proportional to their thickness, while the microtwins in the Gap layer do not agree with that relationship. Also, it is not apparent that those microtwins extending from the interface contain more than seven atom layers. A formation mechanism of those microtwins extending from the interface was proposed and calculations were performed. The theoretical results are in agreement with the experimental measurements.