It is important to calculate accurately the pumping performance of the turbomolecular pump for the purpose of optimising the blade design. In this paper, the pumping performance of the finite-height blade with tip clearance is calculated accurately by the Monte Carlo method in the molecular flow range. To simplify this calculation, velocity components of molecules are given by random numbers. The influence of the two parameters, blade height ratio and tip clearance ratio, is investigated in several blades which are selected as optimum combinations of blade pitch ratio and blade angle. Consequently, the influence of these parameters on the maximum compression ratio is notable on the blade which has small blade pitch ratio and small blade angle. Finally, the results calculated by the present method agree well with the experimental results.