Lean-burn technology is now being reviewed again in view of demands for higher efficiency and cleanness in internal combustion engines. The improvement of combustion using in-cylinder gas flow control is the fundamental technology for establishing lean-burn technology, but the great increase in main combustion velocity due to intensifying of turbulence causes a deterioration in performance such as increase in heat loss and N0x.Thus, it is desirable to improve combustion stability while suppressing the increase in main burn velocity as much as possible (1). It is expected that the fluid characteristics of the in-cylinder tumbling motion that the generated vortices during intake stroke breake down in end-half of compression stroke will satisfy the above requisition.This study is concerned with the effects of enhancing of tumble intensity on combustion in 4-valve S. I. engines. Flow visualization by LLS (Laser Light Sheet) method was applied to the in-cylinder flow field that was varied by means of shurouded valve, and image processing technique using correlation method was utilized for estimating the tumble intensity in motored driving operation. And the breakdwon characteristics due to enhancing tumble were investigated by using numerical simulation code “KIVA”, moreover the results of flame propagation observations and of actual engine test were discussed synthetically to clarify the influence of flow field on combustion.It was recognized that the breakdown process occurs at ealier stage before TDC (top dead center) according as tumble intensity is higher, so the tumble intensity and also turbulence intensity during main combustion would be a same level regardless of initial tumble intensity. Finally, it was confirmed that this flow characteristics can reduce the initital combustion period without significantly increase in main burn velocity.