A tunable modulation format converter based on spectral line-by-line pulse shaper is proposed to realize different format conversions. The pulse shaper works as a format converter by setting its frequency response equivalent to the transform function between two formats. The working principles show that the format converter is suitable for different formats by adjusting its frequency response. Examples of format conversion from return to zero differential phase-shift keying (RZ-DPSK) to on-off keying (OOK) with different data packets and from return to zero (RZ) to non-return to zero (NRZ) are given. The results show that the format converter is not only suitable for different formats but also for random data packets.