Through experimental measurement and theoretical calculation, we have investigated the effect of surface tension, liquid viscosity and the capillary geometry (capillary gradient and tip diameter) on the effective pore throat, and the impact of the effective pore throat on pore resistance to two-phase interfaces formed from fluids with different viscosities and surface tensions. When a two-phase interface flows in channels with varied pore diameter, capillary force is insignificant in the section of the channel with an inner pore diameter greater than the effective pore throat, and the pressure drops in this section depend on capillary tip diameter, rather than the surface tension. Capillary force takes significant effect on the interface only when the pore diameter in a tapered capillary is smaller than the effective pore throat. Effective pore throat depends on fluid surface tension and the capillary geometry, but not on liquid viscosity. The higher the fluid surface tension, the larger the diameter of the effective pore throat. A channel with a large tip diameter or gradient will give a large effective pore throat diameter. Fluid viscosity only affects the magnitude of the resistant pressure drops of fluid flows in constricted capillaries, but does not affect the effective pore throat diameter. The effective pore throat and the pressure profile measured in this study can be explained by the pore contact angle, but cannot be explained by the contact angle on a flat surface of the same materials. (C) 2019 Elsevier Ltd. All rights reserved.