Reports of chronic eosinophilic pneumonia (CEP) after radiation therapy are limited to breast cancer. We herein describe a case of CEP after radiation therapy for lung cancer. The patient was a 65-year-old man who had asymptomatic peripheral blood eosinophilia but no history of asthma or allergy. One month after completion of radiation therapy, chest CT scan revealed infiltrates inside the irradiated area, leading to the diagnosis of radiation pneumonitis. His condition improved after receiving corticosteroids. However, one months after withdrawal of corticosteroids, he noticed cough and shortness of breath with patchy ground glass opacity in the contralateral lung. The WBC count was 9900/μl with 17% eosinophils and bronchoalveolar lavage showed eosinophils of 14%, leading to the diagnosis of CEP. His condition improved after receiving corticosteroids and subsequent tapering without recurrence. Asymptomatic peripheral blood eosinophilia at the initial diagnosis of lung cancer might be a trigger for developing CEP.