Waveguide avalanche photodiodes (WG-APDs) have been developed for a 25-Gb/s operation. The waveguide structure was adopted for the APDs in order to achieve both high-speed performance and high responsivity. First, the dependence of the multiplication and absorption layer thickness on the 3-dB bandwidths of the WG-APDs was theoretically clarified. It was found that introducing an undepleted absorption layer was effective in improving the 3-dB bandwidth without reducing efficiency. The 3-dB bandwidth of the fabricated WG-APD based on the calculation results was over 20 GHz up to a multiplication factor of 7. The responsivities were 0.6 A/W for 1.31 m and 0.75 A/W for 1.55 m wavelength. High reliability of the WG-APDs was also demonstrated.