The 3 omega method for measurement of thermal conductivity of a solid usually consists of a strip heater above a substrate, and the method is based on an approximate solution of the heat conduction problem in which the heater thickness is neglected. We establish a two-dimensional heat conduction model for the heater-on-substrate 3 omega method which takes into account the finite thickness of the heater. Analytical solutions for the finite-heater problem and the vanishing heater-thickness problem are presented utilizing the method of separation of variables. It is found that neglecting the heater thickness is not accurate at low or high frequencies, but works well in between. This frequency range becomes narrower when the heater thickness increases. (C) 2008 Elsevier Ltd. All rights reserved.