Gas turbine nozzle cooling often uses inserts to intentionally distribute cool air through impingement holes to hot spots, especially at leading edge regions of stage-1 nozzles. Owing to the variations of engine operation environment, inlet air filtration systems, upstream component surface coating layers, and oil/air sealing material choices, solid particles could enter the engine from outside or be generated inside from rotor rubbing, seal debris, peeled off coating layers or rusty surfaces, etc. These particles and their agglomerates can be carried by airflow to enter nozzle inserts and clog impingement holes, which may reduce cooling air significantly, resulting in severe engine failures. To reduce the risk of insert clogging, particle separation devices and filtration mesh screens have been implemented in front of nozzle inlets to prevent larger particles from entering. Therefore, designing appropriate nozzle insert hole sizes becomes very critical to let smaller particles pass through and exhaust from the nozzle exit. This experimental study focuses on finding a correlation between the impingement hole sizes of a nozzle insert and their clogged areas caused by seeded fine and medium size particles of Fe2O3 under specific pressure ratios. A nozzle insert was first chosen, and a single row of cylindrical impingement holes was machined at the leading edge of the insert. Measurements were conducted in a pressurized vessel at Reynolds numbers from 7,000 to 62,000 and pressure ratios from 1.01 to 1.10. Results indicate that insert clogging is a strong function of the hole size, particle size, and pressure ratio. The tested particles showed a bimodal distribution of fine and medium sizes, and the medium size particles played a major role in clogging holes. A clogging diagram with 3 zones (fully clogged zone, partially clogged zone, and no clogging zone) is generated from the test data, which can provide important design criteria for sizing the insert leading edge impingement holes to reduce the risk of clogging.