Provision of frequency regulation service with a large number of electric vehicles (EVs) through EV aggregators requires to exchange regulation related messages through communication infrastructures in EV aggregators’ domain. Communication delays in the aggregators’ domain may significantly affect regulation performance. We investigate the effect of communication delays in an EV aggregator’s domain on the regulation performance. We present an underlying communication network for the EV aggregator controlling a group of EVs at a parking lot. The underlying communication network consists of two separate subnetworks: a PLC network between a pair of EV and EVSE and a LTE network between one EV aggregator and a group of EVSEs. In order to evaluate the delay effect on the regulation performance, we propose a co-simulation platform which is composed of an LTE network model and a load frequency control model. For the LTE network, we analyze the potential sources of delays and identify the repeated service request and release procedures as a critical delay source. We implement the identified repeated procedure by using network simulator-3 and evaluate the delay performance of frequency regulation service. We evaluate the effect of the communication delay in the EV aggregator’s domain on regulation performance.