Disclosed is an improved sublimation method for the purification of organic small molecules. The new method features that barriers are applied in the collection region of sublimation tube so that the gas flow path is modified to pass through or bypass the barriers from the heating region to the vacuum pump. The arrangement of the barriers can effectively separate the main product from the impurities. The main product is enriched in a collection region, while the volatile impurities are enriched in an impurity region. This method has been proved to improve the quality of sublimed materials substantially according to the purity measurements and OLED performance tests.