Electronic structure of icosahedral quasicrystals has a pseudogap near the Fermi level. The pseudogap is expected to reveal the large Seebeck coefficient, S, caused by the steep slope of density of state at the Fermi level. Furthermore, the quasicrystals have low thermal conductivity, k, because of the quasi-periodicity. In this work, first, we investigate the thermoelectric properties of AlPdRe icosahedral quasicrystals. The strong composition dependence of S and low k are observed. In a next step, thermoelectric properties of the quaternary AlPdReRu icosahedral quasicrystals that are obtained by replacing Re with Ru in AlPdRe ones have been studied. In the middle of substitution of Ru for Re, the electrical conductivity, s, increases and the peak of S shifts to a higher temperature side. By Ru substitution for the AlPdRe quasicrystal, value of dimensionless figure of merit, ZT, goes up 1.5 times from 0.1 to 0.15. The temperature dependence of electrical properties has been analyzed using a two-band model. According to the result of analysis, the effective mass has peaks at both i-AlPdRe and i-AlPdReRu that reveal the peak values of ZT. We related the behavior of effective mass to change of bond strength of intra- and inter-Mackay Icosahedral clusters.