In this study, we tested the effect of ticagrelor versus clopidogrel on platelet reactivity in patients with minor stroke or transient ischemic attack (TIA). A pre-specified subgroup analysis of a randomized controlled trial was conducted. Platelet reactivity was assessed by thrombelastography (TEG) platelet mapping. Patients were divided into carriers and non-carriers according to the carrier status of CYP2C19 loss-of-function (LOF) alleles. The primary outcome was the proportion of patients with high on-treatment platelet reactivity (HOPR) (defined as maximum amplitude induced by adenosine diphosphate > 47mm) at 90±7 days. Clinical outcomes within 90±7 days were followed up. Among 339 patients, 170 were randomized to ticagrelor/aspirin and 169 to clopidogrel/aspirin. Compared with clopidogrel/aspirin, the proportion of HOPR at 90±7 days in ticagrelor/aspirin was significantly lower (12.2% versus 30.0%, P < 0.001). Ticagrelor/aspirin had a lower proportion of HOPR among carriers (11.0% versus 35.6%, P < 0.001), but not among non-carriers (13.5% versus 22.4%, P = 0.17). Ticagrelor was superior to clopidogrel in inhibiting platelet reactivity measured by TEG platelet mapping among patients with acute minor stroke or TIA, particularly in carriers of the CYP2C19 LOF alleles. Large randomised controlled trials are needed to confirm our findings.