Both automatic and controlled mechanisms have been shown to contribute to the magnitude of the N400 priming effect in adults. It has been proposed that at short stimulus onset asynchronies (SOAs), automatic processes are engaged, while at long SOAs, controlled processes are activated. Here, we explored whether the magnitude of event-related potentials (ERPs) in 18-month-old children are SOA-dependent to further understand the developmental mechanisms underlying semantic priming during early language acquisition. Children were exposed to an auditory semantic priming task in two invariant SOA conditions (1000 ms and 1600 ms). The results showed that the amplitudes of N2, N400 and late posterior negativity (LPN) components were modulated by semantic relatedness, but only those of N2 and LPN were modulated by the SOA length. The amplitudes of the frontally distributed N2 were larger at long than at short SOAs, while the posteriorly distributed LPN was larger over the right hemisphere at the short SOA and more pronounced over the left hemisphere at the long SOA. These findings suggest that both automatic and controlled processes contribute to priming effects in the developing brain, but neural resources underlying these processes might differ