A tunnel magnetoresistance (TMR) sensor is a highly sensitive magnetic sensor workable at room temperature, and its sensitivity is determined from TMR ratio and magnetic anisotropy . Thus, both high TMR ratio and small magnteic anisotropy is required to realize highly sensitve TMR sensor. In this study, we focused on Co-based Heusler alloy Co₂FeAl_xSi_1-x as the free layer of TMR sensors. Magnetic tunnel junctions (MTJs) with these electrodes have been reported to show very high TMR ratio because of their half-metallicity. On the other hand, there is no reports of magnetic anisotoropy of Co₂FeAl_xSi_1-x. In this presentation, we report on systematical investigation of magneto-crystalline anisotropy K₁ of Co₂FeAl_xSi_1-x thin films fabricated by co-sputtering technique.