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This study applies high-speed particle image velocimetry (HS-PIV) and flame image velocimetry (HS-FIV) to show flow fields under the effect of varied swirl ratios in a small-bore optical compression-ignition engine. The base swirl ratio and maximum swirl ratio conditions were applied to investigate structures, magnitude and turbulence distribution of the in-cylinder flow as well as the flow within the flame. For each swirl ratio, 100 individual cycles were measured for PIV analysis at motoring conditions and then another 100 cycles for FIV analysis at firing conditions. The derived flow fields were ensemble averaged to show flow structure evolution while the spatial filtering method was applied to extract high-frequency flow component for the analysis of turbulence distributions. The results showed that the intake air flow generates undefined, chaotic flow fields, which are followed by a gradual production of an asymmetric swirl flow.