We report selective and tunable wavelength converters (STWCs) from an arbitrary wavelength to another arbitrary one, which employ the cascaded second-order nonlinear effect of sum frequency mixing (SFM) and difference frequency mixing (DFM) in quasi-phase matched (QPM) lithium niobate (LN) waveguide devices. Through wavelength conversion experiments using short optical pulses for the QPM-LN waveguide devices having different crystal length, we investigate bandwidth limitation of the QPM-LN-based all-optical STWCs with dual pump configuration. We show that the critical pulse width to be wavelength-converted without waveform distortion is proportional to the length of the LN crystal, and also reveal that those ratio is 1.6 ps/cm. By utilizing this critical value as a performance metric, we successfully demonstrate highly efficient selective and tunable wavelength conversion of 40-Gbit/s data signals using the SFM–DFM cascade in a QPM-LN waveguide device with an optimum crystal length of 5 cm. The 5 cm-long device can be applied to channel-by-channel wavelength conversion in 100-GHz-spaced 40-Gbit/s dense wavelength-division multiplexed systems.