We have previously demonstrated that transcranial alternating current stimulation (tACS) can generate positive effects on multitasking performance and associated neurophysiological measures when it is applied with anti-phase theta band stimulation across bilateral prefrontal cortex (PFC) and a short (1-min) inter-session interval (ISI). However, it is unclear how altering the phase of stimulation and the duration of the ISI might impact positive tACS effects. Here, we investigated the role of tACS parameters in engendering performance improvements by manipulating these two stimulation parameters (i.e. phase and ISI) in two experiments.Repetitive sessions of bilateral PFC theta-tACS were applied with in-phase stimulation + 1-min ISI (experiment 1) and anti-phase stimulation + 5-min ISI (experiment 2) while participants were engaged in a multitasking challenge accompanied by electroencephalography (EEG) data collection.Compared to the control group, in-phase stimulation + 1-min ISI showed an enhancement of multitasking performance coupled with a modulation of posterior alpha (8-12 Hz) and beta (13-30 Hz) activities. However, repetitive sessions of anti-phase tACS + 5-min ISI did not generate significant enhancement in multitasking performance, nor changes in neural oscillatory activities compared to the control group.The results revealed that the previous reported positive tACS effects on multitasking performance are not affected by manipulating the phase of current polarity. Yet, changing the ISI of the stimulation protocol eliminated the previous observed performance improvements. Taken together, these results stress the importance of stimulation protocol for generating positive tACS effects on cognitive function and neural oscillations.