The main purpose of this study was to determine the characteristics of clinical signs, histopathology of pneumonic lung and causative agents in post-weaning pig dealing with respiratory diseases in some pig-farms in the Southern provinces. Age group infected with respiratory diseases was recorded mainly in post-weaning pigs (from 4-8 weeks-old). The clearly respiratory clinic signs appeared in cases of PRDC were difficult breathing, cough, cough with abdominal breathing, and nasal mucus discharge. In addition, the rate of diarrhea and arthritis in these cases was quite high. Two forms of pneumonia identified in the survey were PRDC and interstitial pneumonia. The pathological signs of lung occurred in different forms and accounted for a very high rate, such as: Interstitial pneumonia, bronchitis, pleurisy. In PRDC cases, the highest lung tissue damages were edema/inflammation of septal lobular, subsequent to bronchitis, epithelial necrosis of bronchioles and accumulation of numerous monocytes in alveoli, change of bronchiolitis shape/size, infiltration by neutrophils, fibrin in alveolar spaces, hyperplasia of lymphoid tissue surrounding the bronchioles, and pathological disorderlvascular inflammation. Prevalence of inflammatory lesions in some other visceral organs except lung was very high in this survey, such as: Lymph nodes, spleen, kidney and liver. The initial survey results on the presence of PRRSV, PCV2, CFSV and MH in PRDC lung tissue showed a very high prevalence of 78.9 percent,64.7 percent,36.8 percent and 21.1 percent, respectively.