Gd substitution in DyB 2 C 2 enhances the antiferromagnetic (AFM) interaction and weakens the antiferroquadrupolar (AFQ) interaction. In Dy 0.8 Gd 0.2 B 2 C 2 , two successive transitions occur. By performing resonant X-ray scattering measurements, we confirmed the AFQ ordering temperature TQ=18.0K and the Néel temperature TN=19.5K; TN is higher than TQ in Dy 0.8 Gd 0.2 B 2 C 2 , in contrast to DyB 2 C 2 . There exists an AFM phase between TQ and TN, and the magnetic structure is described by k=[100]. This kind of AFM phase is also observed in HoB 2 C 2 and TbB 2 C 2 , although the magnetic structures of these compounds are long-periodic ones.