Emissions sample probes are widely used in engine and vehicle emissions development testing. Tailpipe bag summary data is used for certification, but the time-resolved (or modal) emissions data at various points along the exhaust system is extremely important in the emission control technology development process. Exhaust gas samples need to be collected at various locations along the exhaust aftertreatment system. Typically, a tube with a small diameter is inserted inside the exhaust pipe to avoid any significant effect on flow distribution. The emissions test equipment draws a gas sample from the exhaust stream at a constant volumetric flow rate (typically around 10 SLPM). The sample probe tube delivers exhaust gas from the exhaust pipe to emissions test equipment through multiple holes on the surface of tube. There can be multiple rows of holes at different axial planes along the length of the sample probe as well as multiple holes on a given axial plane of the sample probe. In a traditional sample probe design, there are multiple planes of holes along the length and several holes evenly distributed on a given plane with a constant hole size. It was observed that the exhaust gas sample composition detected utilizing a traditional sample probe design may not accurately represent the gas composition in the exhaust system especially for samples taken from a larger diameter exhaust pipe.In this study, a systematic numerical investigation was conducted to characterize the mass flow distribution for different emissions sample probe designs used in 3.5\mi and 8\mi exhaust pipe applications. First, the numerical investigation focused on the effects of the number of holes in each axial plane (or row along the circumference on the tube surface) and on the number of rows of holes (along the tube length). Next, the effect of location and orientation of the sample holes, as well as exhaust mass flow rate effects were studied. Then, the effect of sample hole size on sample mass flow rate distribution along the length of the emissions sample tube was investigated. In the end, the sample hole sizes were optimized for both 3.5\mi and 8\mi diameter exhaust pipe applications. Numerical results showed significant improvement in the mass flow rate distribution as the number of holes on a given axial plane in an emissions sample probe tube was reduced from 3 holes to 1. An improvement in the mass flow rate distribution was also found when the number of rows along a column was reduced. Additionally, for a longer sample probe tube in a large diameter exhaust pipe (8\mi), sample probe tube diameter also plays an important role in achieving uniform mass flow rate distribution.