The problem of phase response curves (PRCs) for two neuronal oscillators coupled with chemical synapse is considered in this paper. For small inputs, the PRCs can be computed for a linearized model for the system on the limit cycle. But for an input with large amplitude, the PRCs could be computed by direct method. The shape of phase response curve for single oscillator is depending on the influence of external stimuli, while in neuron networks, neurons are coupled to each other, and the couplings can affect the response of the neuronal oscillators to stimulation. Based on the direct PRCs, we investigate the effects of excitatory and inhibitory synaptic couplings on the PRCs of oscillators, and find that the effects of couplings on phase response of neurons are not only depending on the nature of couplings, but also on the initial phase errors between coupled neurons, which can be explained by the phase difference coupling function.