The current use of the gelam tree is only found in wood processing. On the other hand gelam bark is not optimally utilized so as to produce unused bark waste even though the natural fiber of gelam bark is of high economic value. The purpose of this study was to determine the effect of volume variations on the value of thermal conductivity in gelam fiber wood composite materials with a filler ratio of each volume variation ratio of 10%, 30%, 50% and 70% using polyester resin and alkalinizing skin fiber using 5% NaOH in 2 hours of immersion time and knowing the effect of microstructure on gelam bark fiber composites. Based on the results of the research, the value of thermal conductivity is strongly influenced by variations in the fraction of different volumes in each sample given 5% alkaline treatment which increased from a low fiber composition of 10% to 70% fiber composition from 0,059 W/mºC to 0,107 W/mºC . Composite composition (10% sera - 90% resin, 30% fiber - 70% resin, 50% fiber - 50% resin and 70% fiber - 90% resin) with alkalization 5% shows that there is density and density between the matrix and fiber so that this sample does not have a cavity that inhibits the distribution of heat flow and faster conduction.