The temperature increase due to self-heating and its effect on the photo-mechanical behavior were studied theoretically for photochromic nematic elastomers under light illumination. The sample temperature increase considered here is due to the heat released from the thermally activated back conversion of the azo dyes from the metastable. UV light-induced cis isomers to the stable trans isomers. A temperature equation is derived to include the self-heating effect. Our numerical calculations indicate that there exists a critical environment temperature, below which changes of the NI transition temperature caused by the photo-isomerization dominate the sample contractions and stresses. However, at higher environment temperatures, a sample temperature increase can produce larger contractions and stresses in mono-domain liquid crystal elastomers. The thermal effect is particularly strong near the initial nematic-isotropic transition temperature. Thus, the experimental reported photo-mechanical behavior is a coupled phenomenon of the thermal- and the photo-mechanical effects in general. More accurate thermal-mechanical experiments need be contacted to track this effect. (C) 2009 Elsevier B.V. All rights reserved.