Abstract: The hourglass has been used as a historical time-keeping tool and its physical principles are still applied in all aspects of life. In this paper，different particle materials are studied. Under the condition that the diameter of particles is extremely small and the specific surface area is sharply increased，the flow rate of particles is positively correlated with their diameters when keeping the hole of the same diameter. At the same time，the maximum stability angle is measured and the influence of factors，such as opening diameter and inclination angle on the flow velocity is further explored.

Key words: fine particulate matter, Beverloo formula, hourglass, specific surface area, maximum stability angle