Massive lung gangrene is a rare but very rapidly progressing fatal complication of lobar pneumonia. Etiologic agents are Klebsiella pneumoniae, Pneumococcus and Aspergillus, etc. Chest X-ray shows firm consolidation of the involved pulmonary lobe and bulging fissure due to the volume expansion of involved lung. CT-scan shows extensive lung parenchymal destructions with multiple small cavitary lesions. Recommended treatment is the early surgical intervention combined with antibiotics. Without surgical intervention, lung gangrene is known to progress toward sepsis, multiorgan failure, and high mortality. We report two cases of rapidly progressing massive lung gangrene by Klebsiella pneumonia treated by the resectional surgery.