Emerging evidence from studies of Huntington disease (HD) pathophysiology suggests that huntingtin (htt) and its associated protein HAP1 participate in intracellular trafficking and synaptic function. However, it is largely unknown whether AMPA receptor trafficking, which is crucial for controlling the efficacy of synaptic excitation, is affected by the mutant huntingtin with polyglutamine expansion (polyQ-htt). In this study, we found that expressing polyQ-htt in neuronal cultures significantly decreased the amplitude and frequency of AMPAR-mediated miniature excitatory postsynaptic current (mEPSC), while expressing wild-type huntingtin (WT-htt) increased mEPSC. AMPAR-mediated synaptic transmission was also impaired in a transgenic mouse model of HD expressing polyQ-htt. The effect of polyQ-htt on mEPSC was mimicked by knockdown of HAP1 and occluded by the dominant negative HAP1. Moreover, we found that huntingtin affected mESPC via a mechanism depending on the kinesin motor protein, KIF5, which controls the transport of GluR2-containing AMPARs along microtubules in dendrites. The GluR2/KIF5/HAP1 complex was disrupted and dissociated from microtubules in the HD mouse model. Together, these data suggest that AMPAR trafficking and function is impaired by mutant huntingtin, presumably due to the interference of KIF5-mediated microtubule-based transport of AMPA receptors. The diminished strength of glutamatergic transmission could contribute to the deficits in movement control and cognitive processes in HD conditions.