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Lean homogeneous charged compression ignition (HCCI) combustion achieves clean and high thermal efficiency, however the operating load range is limited by steep combustion and NOx emission near the high load. Therefore, it is necessary to switch the combustion modes of HCCI and spark ignition (SI) under minimized torque gap for practical use, but the large gap between the combustible air-fuel mixture of these two combustions makes it difficult to be accomplished. In this study, a new switching assist combustion using high-pressure fuel injection was applied to resolve this issue. As a result, the gap between HCCI and SI combustions was connected successfully.