In this paper, the circuit models of a waveguide photodiode (WGPD) and its submodule were investigated, and the O/E characteristics of a WGPD submodule are examined. Test structures of the WGPD and WGPD submodule were fabricated and microwave return loss (S11) was measured and compared with simulated data to validate the circuit models. With the established submodule model, optical to electrical (O/E) characteristics were measured and compared with the modeled data to analyze the effects of model parameters on the submodule performance. Based on the results, it can be concluded that the suggested submodule model can explain the characteristics of the submodule performance. In addition, parasitic components that originated from the ribbon bonding block can crucially impact on the performance of WGPD submodule.Index Terms-Circuit model, optical receiver module, optical to electrical (O/E) response, waveguide photodiodes (WGPDs).