GaN/AlxGa1-xN superlattices (SLs) with different period thicknesses tp were grown as interlayers between GaN and AlyGa1-yN epilayers. The effect of threading dislocations (TDs) blocking became more evident with increasing tp. Transmission electron microscopy analysis shows that TDs are inclined to be bended in SLs and terminated in GaN wells as a result of strain. X-ray diffraction measurement also validated that GaN wells played a more important role as a TDs filter. The blocking of TDs in SLs resulted in an abnormal decrease in relaxation factors R(SLs) with increasing tp.