Interleukin 18 (IL18) is known to induce the expression of interferon‐γ (IFNG), but its effects on T cell proliferation and costimulation are not completely understood. In this study, we demonstrate that ectopic expression of IL18 in CART cells caused significant T cell proliferation in vitro and in vivo, and enhanced antitumor effects in xenograft models. Moreover, IL18 mediated T cell expansion required neither tumor antigen nor CAR expression, and produced severe GVHD in NSG mice. Furthermore, recombinant IL18 costimulated IFNG secretion and proliferation of anti-CD3 beads treated T cells. Interestingly, IL18 costimulation could expand purified CD4 T cells, but not CD8 T cells. However, CD8 T cells proliferated greater than CD4 T cells in magnitude within bulk T cells, suggesting CD4 help effect was involved. Using CRISPR/Cas9 gene editing, we confirmed that IL18‐driven expansion was both TCR and IL18 receptor (IL18R) dependent. Importantly, we demonstrated that TCR‐deficient, IL18‐expressing CD19 CART cells exhibited remarkable proliferation and persistent antitumor activity against CD19‐expressing tumor cells in vivo, without eliciting any detectable GVHD symptom. Finally, we describe APACHE T cells, a novel strategy for coupling IL18 expression in CART cells to antigen stimulation, thereby limiting potential toxicity associated with persistent IL18 production. In sum, our study supports human IL18 as a T cell costimulatory cytokine for fueling CART therapy.