Abstract: There is velocity difference in plunger and liquid filling between plunger and tubing during the up-travel of plunger. Leakage ratio of liquid effect the selection of plunger size and design of plunger gas lifting technique, suppose that plunger is concentric with string in up-traveling, based on mass and momentum-conservation principles, a new model about leakage ratio of plunger gas lifting was established, and liquid velocity distribution, leakage volume and leakage ratio equation were also developed. Calculated leakage volume using Hernandez experiment data, got curves of liquid leakage and slug height of liquid up plunger with steam injection pressure. The model is scientific and useful for plunger gas lifting design.