This study aimed to evaluate impacts of ventilation designs on indoor airborne transmission for mixing ventilation (MV) focusing on ventilation effectivness. Different flow rates (8 to 40 l/s), exhaust positions (high and low), source locations and effects of air cleaning were tested, and particle concentrations with different size fractions were measured. The results showed that the ventilation effectiveness was decreased with an increased flow rate, which was reduced from about 1.15 (with 8 l/s) to 0.9 (with 40 l/s) for a standard MV configuration. With 30 l/s the effectiveness was about 1. This trend was due to stratification with low flow rates and short-circuit related to high flow rates. Although lower flow rates showed greater ventilation effectivness at the compared locations, higher flow rates would always provide better dilution. This suggested that a balance between the contaminant dilution and removal should be considered when increasing flow rates to enable sufficient and effective ventilation. Moving the exhaust from the ceiling to floor level was less effective due to short-circuit. Placing the source close to, and under the exhaust helped to remove contaminants more efficiently. Air cleaning added significant impacts on ventilation to reduce particles for the supply flow rate of 15 l/s