We proposed and demonstrated an all-optical format conversion from return-to-zero quadrature phase shift keying (RZ-QPSK) to non-return-to-zero QPSK (NRZ-QPSK) at 40 Gb/s using a half bit delay interferometer (DI). Due to the constructive interference in the DI, the format conversion was achieved with the phase information preserved. Q penalty for the format conversion was less than 1.8 dB for I and Q data.