From the view of signal processing in the frequency domain, a scheme for the return-to-zero (RZ) to nonreturn-to-zero (NRZ) signal data format conversion based on a tunable microwave photonic filter is analyzed and demonstrated. Theoretically, the spectral characteristics of RZ and NRZ baseband signals and microwave photonic filter characteristics are analyzed. It is found that RZ to NRZ data format conversion can be successfully realized by a low-pass filter on the RZ signal with strong suppressing the clock component of the RZ data. In the experimental demonstration, a 10 Gbit/s NRZ signal is successfully converted from a RZ signal by utilizing an two-tap microwave photonic filter with two different light sources and intensity modulation. Both waveform and spectrum show the signal has good quality. In addition, signals with various repetition rate can be well processed by simply adjusting some parameters of the filter. It is shown that this scheme is very simple and flexible.