[Purpose] The purpose of this study was to identify changes in pulmonary function and pulmonary strength according to time of day. [Subjects and Methods] The subjects were 20 healthy adults who had no cardiopulmonary-related diseases. Pulmonary function and pulmonary strength tests were performed on the same subjects at 9:00 am, 1:00 pm, and 5:00 pm. The pulmonary function tests included forced vital capacity (FVC), forced expiratory volume in 1 second (FEV₁), and forced expiratory flow between 25 and 75% of vital capacity (FEF_25–75%). Pulmonary strength tests assessed maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP). [Results] FEV1 showed statistically significant differences according to time of day. Other pulmonary function and pulmonary strength tests revealed no statistical differences in diurnal variations. [Conclusion] Our findings indicate that pulmonary function and pulmonary strength tests should be assessed considering the time of day and the morning dip phenomenon.