Dimensionless thermoelectric figure of merit $ZT$ is investigated for two-dimensional organic conductors $tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, $tau$-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ and $tau$-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ ($y le 0.875$), respectively. The $ZT$ values were estimated by measuring electrical resistivity, thermopower and thermal conductivity simultaneously. The largest $ZT$ is 2.7 $times$ 10$^{-2}$ at 155 K for $tau-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, 1.5 $times$ 10$^{-2}$ at 180 K for $tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ and 5.4 $times$ 10$^{-3}$ at 78 K for $tau-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, respectively. Substitution of the donor molecules fixing the counter anion revealed EDT-S,S-DMEDT-TTF is the best of the three donors to obtain larger $ZT$.
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