The present work is carried out to reduce the leakage through the labyrinth seal by optimizing the tooth profile and operating clearances.        The effects of pressure ratio, clearance, tooth width and height, and number of throttles upon the leakage through a straight-through labyrinth seal for in­compressible flow were experimentally investigated. Measurements in a developed test section over wide geometric factors and flow conditions were carried out.     The experimental results indicated that the pressure ratio had significant effect on the flow coefficient. The flow coefficient decreased with increasing the pressure ratio. As the clearance increased the flow coefficient increased. However, the flow coe­fficient was increased as the clearance, became very small. The flow coefficient reached a maximum value at tooth width to clearance ratio of 2. The effect of tooth height on the flow coefficient was found negligible for tooth height to clearance ratio higher than 2. The flow coefficient decreased as the number of throttles increased, then attained constant value when the number of throttles became larger than seven. The observed trends of the results for incompressible fluid through straight-through labyrinth seals can be used to establish some general geometric constraints for minimizing the leakage.