Influence of the inter-ring crevice, the volume between the top and second piston rings, on unburned hydrocarbon (UHC) emission was experimentally and numerically investigated. The ultimate goal of this study was to estimate the level of UHC emission induced by the blow-up of inter-ring mixture, i.e., unburned gases trapped in the inter-ring crevice.In the experiments, the inter-ring mixture was extracted to the crankcase during the late period of expansion and the early period of exhaust stroke through the engraved grooves on the lower part of cylinder wall. Extraction of the mixture resulted in the significant reductions of UHC emission in proportion to the increments of blowby flow rate, without any losses in efficiency and power. This experimental study has confirmed the importance of inter-ring crevice on UHC emission in an SI engine and established a relationship between the inter-ring mixture and UHC emission.A physical model was constructed to predict the gas flows through the piston ring pack and used to interpret the phenomena related to the results of experiments. Amount of the inter-ring mixture returning to the combustion chamber after the exhaust valve open was calculated and converted to the corresponding UHC emission using the relationship between the inter-ring mixture and UHC emission in the experiments. Calculated level of UHC emission caused by the inter-ring crevice was 10 % ∼ 30 % of the entire UHC emission over a range of speeds (1250 rpm ∼ 3500 rpm) and loads (185 kPa ∼ 556 kPa bmep), which showed maximum at 2500 rpm, 432 kPa bmep. This is a condition most frequently operated by users.