The Combine is necessary equipment in the agricultural industry. The components of the Combine are worked for a long time under inferior environmental condition. Especially reaping knife is worked in high frequency domain, and submit fatigue load while use. So, fatigue test was performed to obtain the S-N curve of real component, and load history is measured through field test. The local stresses due to these loads have been calculated by FEM. These results have been used as the input values for the multi-axial fatigue analysis of real components. For the assessment of multi-axial fatigue damage, the critical plane methods have been employed. The used parameters of critical plane methods are Morrow’s, Smith-Waston-Topper’s, and Brown-Miller’s, those were modified for the high cycle fatigue region within elastic behavior. In addition, design improvement is performed about shape of reaping knife to increase the endurance limit, and durability test for improved knife is performed. It is found that the fatigue life of improved reaping knife is increased, and durability test result show that life of reaping knife is increased enough.