An insulated metal substrate (IMS) is a circuit board comprising an insulating layer on a metal base plate. The insulating layer is made from epoxy resin incorporating dense ceramic fillers. The substrates are used in applications where electric parts generate intense heat. It is expected that the insulating layer has higher thermal conductivity as the use of an IMS is expanded. Therefore, the influence of percolation on the equivalent thermal conductivity (ETC) of an insulating layer is considered. The Effect of the volume fraction of ceramic filler on the ETC of insulating layer in IMS is investigated. The ETC as a function of volume fraction of filler is estimated. Based on these experimental and numerical results, an ETC of a filler is evaluated. The ETC of an irregular filler is presumed smaller than that of a spherical filler. It is thought that the control of filler size and shape is important for the formation of high thermal conductivity of an insulating layer. In addition, an improved equation for the ETC of IMS is proposed. The predictive values from the equation for an improved IMS agree with experimental results.