This study investigates experimentally and numerically the influences of FOUP door opening speed (V(subscript door)) and the pressure difference (dP) on the dynamic distributions of the averaged nondimensional particle concentration in the FOUP (C(subscript ave)), which is defined as averaged particle concentration in the FOUP (c(subscript ave)) divided by the particle concentration in the clean room (c(subscript CR)). Here dP is the difference between the mini-environment pressure and the clean-room pressure. Results show that when V(subscript door) lies between 0.05 m/s~0.15 m/s and dP within 0.3 Pa-12.7 Pa, C(subscript ave) proportional to 1/2ρV(subscript door) 2 and inversely proportional to dP. This can be expressed by a stepwise multiple regression equation: C(subscript ave)=4.56×10^(-1) (1/2ρV(subscript door)^2)-4.6×10^(-4) dP+4.96×10^(-3).