The switching process of transistor Schmitt circuit was studied in this paper. Concrete analytical expressions of the switching process have been derived. Using these expressions numerical calculations were carried out for two types of commonly used transistor Schmitt circuit. Experiments were carried out to check the theoretical calculations. Experimental results fit the theoretical calculations quite well. From this study, some rules for selection of the transistors and circuit elements were obtained. The results of calculation in this paper may be used to estimate the switching speed of Schmitt eircuit in the early design stage.