In this paper, a behavioral model of threshold voltage for normally-off (enhancement mode) AlGaN/GaN based tri-gate HEMT is proposed. AlGaN/GaN based tri-gate HEMT devices have additional sidewall gates and show threshold voltage variation with decreasing device width. The proposed model captures strain relaxation with reduction in device width, which is one of the primary reason for change in Vth in AlGaN/GaN tri-gate devices. Model shows excellent agreement with state-of-the-art experimental and simulation data. © 2014 IEEE.