To increase the intake mass flow rate on a Downsizing diesel engine, the tilting axis of intake valve was chosen to enlarge the intake valve diameter Thus cylinder head had to be redesigned to meet this demand. Geometry of cylinder head made effect in organization of in-cylinder flow, fuel-air mixing quality and further combustion performance. In this study, two cases of combustion chamber structure formed by one-cut and four-cut processing methods respectively were studied. 3-D CFD was a convenient and cheap tool to explore effects of these two styles of cylinder head on the combustion process. This work was done on the ESE Engine module of commercial CFD software AVL FIRE and operated by comparisons of these two cases on the development of swirl ratio, equivalence ratio distribution and combustion characteristics. The results show that, in term of swirl ratio, two cases reflect a similar growing trend, but a weaker swirl (down by 9.1%) is formed in the one-cut case; resulting from its weaker swirl, this case delivers a 0.6CA longer in combustion delay and a 0.5CA later in combustion center than the four-cut case under the same injection timing; however, with the same injector protrusion height, a better diffusion of mixture can be found in the one-cut case contributed by its pent-roof shape, which enhances the increase of portion of heat release of premixing phase and the decrease by 0.9CA in combustion duration, but the combustion efficiency of these two cases are almost the same.