We present an intrinsic error in differential 3-omega measurement method due to two-dimensional heat spreading effect in the upper layer of the target film. By measuring thermal conductivities of 300 nm PECVD-grown silicon dioxide thin films with various thicknesses of upper layers, significant heat spreading effect is observed. Also, analytical modeling regarding apparent thin film thermal conductivity is conducted for verification of experimental results. Experimental results as well as analytical results show that the measurement error tends to increase with thickness of upper layer due to two-dimensional heat spreading effect. (C) 2008 Elsevier B.V. All rights reserved.