Lung nodules are generally defined as high-density shadows of various shapes, usually within 3 cm in diameter, found on imaging. And lung nodules smaller than 0.5cm are defined as micro nodules. Lung nodules of 0.4cm can be observed clinically and followed up regularly if the patient has no symptoms such as cough or chest pain. The first follow-up period is 3 months, and if there is no change in the nodule, it is recommended that it be followed up again in 1 year. 1 year later, if there is still no change in the follow-up, it means that the nodule has basically stabilized, so there is no need to pay too much attention to it, and regular annual physical examination will be sufficient for the follow-up. If the nodule is found to be gradually increasing in size or suspicious in shape, timely treatment, such as surgical resection, is required. Generally, malignant nodules are irregular in shape on CT imaging, with generally unclear edges, some peripheral burrs, and generally larger diameters. If malignant nodules are highly suspected, early surgical resection is recommended regardless of the size of the nodule. In conclusion, a 0.4cm lung nodule is not a big one, so it is recommended to follow the guidance of professional doctors and standardize the treatment.