The airbag pressure waveform is the basis of the occupant restraint provided by an airbag and greatly influences the level of restraint performance achieved. The characteristics of the gas flowing into and out from the bag were analyzed experimentally and theoretically in order to make clear the factors determining the airbag pressure waveform when the bag was inflated in isolation. The results clarified the mechanism by which different factors affect the bag pressure waveform, including the gas flow characteristics from an inflator into an airbag, gas exhaust characteristics from a bag vent and various bag specifications such as volume. A simple numerical analysis model and an experimental model were prepared to analyze the relationship between the bag pressure waveform and occupant restraint performance. Analyses carried out with these models revealed the relationship between the airbag pressure waveform and deceleration of an impactor corresponding to the occupant. They also clarified the impactor restraint mechanism of the airbag. These results have indicated one direction to take in developing airbag specifications which will reduce impactor (i.e. occupant) deceleration.