We investigate the microscopic optical force density distributions respectively inside a subwavelength-diameter (SD) fiber with fl at endface and inside one with oblique endface by using a finite-difference time-domain (FDTD) method. Optical force density distributions at the fiber endfaces can now be readily available. The complete knowledge of optical force density distributions not only reveal features regarding the microscopic near-field optomechanical interaction, but also provide straightforward explanations for the sideway defl ections and other mechanical motions. Our results can provide a useful reference for better understanding the mechanical infl uence when light transports in a microscale or nanoscale structure and for developing future highly-sensitive optomechanical devices.