Quantum definition based threshold voltage calculation of Gate-All-Around Junction Less Nanowire Transistor has been presented in this work. Employment of this approach has not yet been achieved on GAA-JLNT even though similar determination was previously established for TG FinFETs and GAA-MOSFETs in recent literature. The self-consistent solver, which takes wave function penetration and other quantum mechanical (QM) effects into account, has been utilized here to establish the Capacitance-Voltage characteristics, using which the threshold voltage has been calculated. Effect of silicon channel thickness on threshold has been studied and a modification of classical analytical formula is proposed based on a fitting parameter, using the extracted threshold voltages.