An FIR filter of a DS-CDMA UWB modem transmitter and a control method thereof are disclosed. The FIR filter includes an LUT control device for outputting a resultant value of "0" to all adders if data values corresponding to upper three chips H2, M2 and L2 and lower three chips H, M and L are "000000", and discriminating which group between a first group and a second group the upper/lower data values belong to if the upper/lower data values are not"000000". The LUT control device provides upper or lower LUT values to the adders using the upper or lower LUT values as they are, or converts the upper or lower LUT values into 2's complements and provides the converted values to the adders according to the discriminated first or second group.