<p>Correlated spiking has been widely found in large population of neurons and been linked to neural coding. Transcranial alternating current stimulation (tACS) is a promising non-invasive brain stimulation technique that can modulate the spiking activity of neurons. Despite its growing application, the tACS effects on the temporal correlation between spike trains are still not fully understood. In this study, we use a pair of unconnected two-compartment model neurons of the integrate-and-fire (IF) type to simulate the correlated spike trains driven by shared fluctuating dendritic inputs and exposed to weak alternating electric fields. Our results show that the output correlation increases with field intensity, but increases and then decreases with field frequency, displaying thus a frequency resonance. Through varying somatic and dendritic morphologies, we demonstrate that morphological differences between the soma and dendrites fundamentally shape the correlation-frequency resonance, with more pronounced differences yielding stronger resonance effects. Moreover, the anti-phase sinusoidal modulations induced by tACS at the soma and dendrite promote this correlation-frequency resonance, particularly when dendritic fluctuations exhibit a large mean value. We further examine the tACS effects on output correlation in biophysically and morphologically realistic pyramidal model neurons, revealing similar patterns to those observed in the two-compartment models. Our findings provide new insights into how tACS modulates the correlated spike trains and highlight the critical role of morphological differences between the soma and dendrites in determining the frequency-dependent output correlation. These predictions should be taken into consideration when understanding the tACS effects on population correlation and population coding.</p>