Avalanche photodiodes (APDs) are critical photodetection devices for high sensitivity and high-speed sensing. Great efforts in bandgap engineering and impact-ionization engineering have been put to achieve high gain-bandwidth products. Plasmonic optical antenna (POA) techniques can change the near-field distributions of the illumination light and thus provide an effective approach to achieve simultaneous enhancement of the gain and the detection speed. We report a metallic circular disk POA array-enhanced APD, where the circular disk POA array was fabricated on top of the p-doped side of the GaAs p-i-n APD. The electric field (E-field) distribution were simulated. The gain and the frequency response of the APD POA-enhanced APD were measured and compared with a reference APD without the POA array. The POA-enhanced APD shows a simultaneous enhancement in the gain and the detection speed.