A method of controlling a filter coefficient of a continuous time filter of a data storage device is provided. The method of controlling filter coefficient includes positioning a magnetic head at a first reference height from surface of a magnetic disk and outputting a plurality of CSM values by measuring a channel state from a read signal output from the magnetic head when changing a cut-off frequency of the continuous time filter, positioning the magnetic head at a second reference height spaced from the first reference height and measuring a bit error ratio from a read signal output from the magnetic head positioned at the second reference height according to at least one CSM value selected among a plurality of CSM values, and controlling a filter coefficient value to determine a cut-off frequency of the continuous time filter according to a measured bit error ratio.