The bleeding (internal transport) process of additives in a polypropylene film under atmospheric pressure was investigated. The experimental results were explained more precisely by a new model assuming the two-step transport between the amorphous and crystalline regions. The diffusion coefficient of a higher fatty acid such as behenic acid (docosanoic acid) in isotactic polypropylene film and that of higher fatty acid amides such as erucamaide (13-cis-docosenamide) in ethylene copolymerized polypropylene film were determined at 40 and 50 degrees C respectively. The difference between the diffusion coefficients of three slip agents in a polypropylene film at 50 degrees C was explained using a molecular dynamics simulation in which self-association of the slip agent molecules by hydrogen bonding was considered.