This paper presents an all-digital IR-UWB transmitter with high energy efficiency and flexible pulse shaping for OOK modulation systems. A digital amplitude modulation (AM) technique is proposed to enable configurable pulse shaping with a single digital power amplifier (DPA) for low power design. The proposed transmitter implemented in 65nm CMOS achieves the maximum pulse energy of 3pJ/bit and occupies an active area of 0.08mm2, meeting the FCC spectrum mask in 3-5GHz UWB band with a PRF of 32MHz.