The orifice-flow system has been commonly used in the calibration and measurement of the vacuum systems. Such a system usually consists of a vacuum chamber in which two cylindrical or spherical halves separated by a plate with the orifice in the center. For the application of the orifice-flow system, calculating the correct conductance of the orifice is the most important thing to do. However the actual flow condition inside the chamber, such as the gas density distribution and gas flux distribution, also restricts the accuracy of the calibration or measurement. In this paper the flow field inside a cylindrical vacuum chamber of an orifice-flow system is investigated using the direct simulation Monte Carlo (DSMC) method. A known gas flow is introduced to the upper chamber through a port in the center of the top plate. We simulate cases in which the outlet boundary is set to be in vacuum or given a known pressure that represents a vacuum pump. The observation is focused on the pressure distribution in these cases. In each case, baffles are added in the upper and lower chamber after the flow reaches a steady state. Baffles in the orifice calibration system are used to improve the uniformity of the gas density distribution and the gas flux distribution. Therefore the variations of the pressure distribution and the velocity field in the chamber after baffles are added are particularly investigated.