Two strain relaxation processes have been observed in AGaN layers grown on thick AlN templates. In Process I, a-type threading dislocations (TDs) with b = 1/3(112̄0) from the AlN underlayer are inclined away from the [0001] axis toward the (11̄00) directions when they enter the AlGaN film, forming dislocation line projection perpendicular to the Burger vector. In Process II, a + c-type TDs from the AlN underlayer with Burgers vector of b = 1/3(112̄3) glide on (01̄11) planes when they enter the AlGaN film to generate interfacial misfit dislocations lying along the (2̄110) directions at the AlGaN/AlN interface. © 2010 The Japan Society of Applied Physics.