Both of avian influenza (AI) and Newcastle disease (ND) can cause mild to severe diease in poultry. In this study, clinical signs, macro, and micro lesions were studied. Eighteen six-week-old SPF chicks were divided into 4 groups (E1, E2, E3 and C1) and housed in different rooms of the isolation facility at CAVAC (Daejeon, Korea). The control group (C1) of 3 chicks was housed separately as uninoculated. Experimental groups (E1, E2 and E3) challenged with H9N2 and/or NDV. E1 group was challenged with 0.1 mL A/Kr/Ck/01310/01 (H9N2) $10^{5.6}$ $EID_{50}$ by intranasal, E2 group was challenged with 0.5 mL Kyojeongwon (KJW) $10^{5.0}{sim}10^{6.0}$ $ELD_{50}$ by intramuscular, and E3 group was challenged with 0.1 mL A/Kr/Ck/01310/01 $10^{5.6}$ $EID_{50}$ by intranasal and 0.5 mL KJW $10^{5.0}{sim}10^{6.0}$ $ELD_{50}$ by intramuscular 7 days after H9N2 challenge. In clinical signs and gross findings, E1 group showed 0% mortality, anorexia, and hemorrhage of proventriculus and thymus, E2 group showed 100% mortality within 3~5 days after challenge, anorexia, green diarrhea, hemorrhage of proventriculus, proximal esophagus and thymus, enlargement of kidney, and bronze liver, and E3 group showed 100% mortality within 24~36 hours after NDV challenge, depression, anorexia, green diarrhea, hemorrhage of proventriculus, spleen, and lung, enlargement of kidney, and reduction of thymus size and number. In histopathological examination, E1 group showed depletion and necrosis in bursa of Fabricius, thymus, and spleen, and E2 and E3 group showed severe lymphocyte depletion and necrosis with destruction of lymphoid organ structures. In conclusion, co-infection of H9N2 with ND virus causes acute disease with high mortality than single infection and the pathologic lesions were more severe.