The various composition of particle size in the lost circulation material are tested in a simulated formation under laboratory conditions to evaluate their sealing performance. The well model is made of a double pipe for testing with circulated mud treated with the lost circulation material. The high permeable formation model is packed with 10.3mm beads packed hexagonally with a theoretical porosity and pore size of 39.5% and 1.55mm respectively. Screening walnut shells are used as the lost circulation materials. Several particles having maximum size of the half and full pore size of the formation model are selected and the lost circulation materials are mixed in mud at forty-five grams of the material per liter of mud. These muds to which each lost circulation material has been added, are tested under 98-834kPa pressures. The experimental results indicate that a lost circulation material containing 20% maximum size particles of full size to the formation pore size is the optimum size composition to seal the high permeable formation.