The prevalence of anxiety disorder in Indonesia increases significantly every year. It is worth special attention, especially in facing the uncertainty during pandemic. Acceptance and Commitment Therapy (ACT) is one of the third-wave cognitive behavior therapy (CBT), usually used for anxiety disorder treatment. In this research, I want to examine the effectiveness of Acceptance and Commitment Therapy (ACT) to decrease the level of Generalized Anxiety Disorder (GAD), taking General Anxiety Disorder-7 (GAD-7), Geriatic Anxiety Inventory (GAI), and Beck Anxiety Inventory (BAI), as the standard. It uses a meta-analysis techniques processed by statdirect trial version 2.0. It involves 17 experimental journals of ACT and GAD (n experiment group = 574; n control group = 513). The assessment of ACT and GAD bases itself on random effect size (I2 = 83.3%), without any publication bias (-3.88). The result shows that ACT is still effective to decrease the level of GAD (d= -.66; medium effect size). The result concludes that the ACT needs the assistance from other interventions. It also suggests research replication to multiply the references related to the effectiveness of ACT.