An adjusting structure for adjusting amount of lubricant applied to a linear guideway comprises a self-lubricating oil tank, a positioning board and at least one adjusting board. The self-lubricating oil tank is fixed on the slide block, and the self-lubricating oil tank has an oil-storage space and a lubricating unit. The adjusting board and the positioning board are superposed and arranged between the oil-storage space and the lubricating unit. The displacement of the adjusting board can change the size of the aligned apertures of the positioning board and the adjusting board, and can precisely adjust the amount of lubricating oil transmitted from the oil-storage space to the lubricating unit. Therefore, the present invention can use the adjusting board to change the lubricating effect at any time, it not only can save lubricating oil but also can improve the lubricating effect.