A mathematical model for micro thermal analysis is developed based on heat conduction equation. It interprets the principle of micro thermal analysis by relating its signals directly to established parameters of materials, such as thermal conductivity, specific heat etc. Based on the theory, a new instrument is designed employing three components of electrical current to drive a thermal probe. A high frequency component is used to measure probe temperature; a low frequency and a dc component are used for temperature control. Thermal properties of materials are measured by monitoring ac temperature response to a known ac driving power and monitoring dc power consumption for a known dc temperature input. With the new design, thermal information of a sample can be separated from the artifact caused by a measuring probe. Glass transitions of polymers can be easily identified from both dc and ac signals. (C) 2004 Elsevier B.V. All rights reserved.