The critical properties of five compounds, including propanal, butanal, 1-pentanal, 2-methel butanal and trimer of ethanal, were determined for the first time by a new capillary quick-flow method. The apparatus was improved with a capillary tube and checked with hexane and 1-heptene as standard reagents. The experimental results proved that the determination of critical properties by quick-flow method with a capillary tube apparatus was successful.