We synthesized 9 kinds of FeCl3/[C(4)mim][Cl] (iron (III) chloride with 1-butyl-3-methylimidazolium chloride) ionic liquids from a mole composition ratio of 1/1.7 to 1.5/1. The conductivities and densities of these FeCl3/[C(4)mim][Cl] ionic liquids are measured in the T = (293.15 to 343.15) K range respectively. An extreme increase in the ionic conductivity of tested FeCl3/[C(4)mim][Cl] ionic liquid is found when the mole composition ratio of FeCl3 to [C(4)mim][Cl] (x) is less than 1/1, while the conductivity smoothly decreases with the mole composition ratio of FeCl3 to [C(4)mim][Cl] greater than 1/1. Interestingly, the viscosity of FeCl3/[C(4)mim][Cl] ionic liquids decreases as increasing the mole ratio of FeCl3 in the whole mole composition range. As 1/1.7 <= x <= 1/1, the viscosities decrease extremely, while the trend turns smoothly as x >= 1. As increasing the mole composition ratio of FeCl3 to [C(4)mim][Cl], the thermal coefficient of expansion of FeCl3/[C(4)mim][Cl] ionic liquid gets larger, but the opposite trend is observed for crystal energy at T = 298.15 K according to Glasser's theory. (C) 2016 Published by Elsevier Ltd.