Two-dimensional (2D) numerical models are frequently adopted to investigate combustion and thermal performances in rectangular micro-channels for micro-thermophotovoltaic and thermoelectric devices. However, large error may exist by applying a simple 2D model. In the present work, the outer wall temperature distributions predicted by 3D model and simple 2D model were compared. The results showed that the maximum relative error of the simple 2D model depends significantly on the aspect ratio (alpha) of the micro-channel. To be specific, the maximum relative error was >30% for alpha = 1 and > 10% for 2 <= alpha <= 4. However, it was <5% for alpha >= 9. A new 2D model was proposed to modify the underestimated heat loss ratio. The new computational results demonstrated that the maximum relative error of alpha = 1 decreased to 8.07% and for micro-channels with alpha >= 2, all the maximum relative errors are <5%. In summary, the modified 2D numerical model can achieve a satisfactory prediction with low computation cost. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.