The heart is the most important muscular organ in human's body, which pumps blood through the arteries to supply the body with oxygen and nutrients. The heart is supplied by the coronary blood vessel; therefore, the effect of aneurysm and stenosis in left coronary artery on the velocity of blood, wall shear stress of artery and mass flow rate of blood have been investigated in this study. The simulation program (ANSYS Fluent) was used to execute the numerical study. Typical geometry of left coronary artery and physiological parameters of human blood values were obtained from measured values reported in literature. The problem of the effect of the aneurysm and stenosis on the human blood stream has been solved numerically under three conditions, healthy artery and two infected cases (30%, 50%) percentage of aneurysm in left main stem (LMS) and stenosis in left anterior descending (LAD). It has been shown, in stenosis region that the velocity of blood will suffer fast flowing and an increase in the shear stress on the artery wall, in contrast with the aneurysm case, blood velocity becomes slow and low wall shear stress. Also irregularity was shown in mass flow rate of blood in the left coronary artery which suffers from aneurysm and stenosis compared with healthy artery.