This research is aimed to study the transient behavior of air flow through the intake manifold of a heavy duty diesel engine according to the step of valve opening. The numerical study of air flow through the intake manifold was done on the 6SD-1 Isuzu engine. Two numerical techniques, finite element and finite volume methods were used in comparison and tendency confirmation of air flow characteristics inside the manifold. The transient air flow was studied at the engine speed of 1,000 rpm, 1,500 and 2,000 rpm. The numerical results from both numerical techniques agree exceptionally well with each other. The transient behavior of air flow according to the step of valve opening is well illustrated.