The development trend of miniaturization and integration of high-performance electronic devices brings a difficult problem in heat dissipation. Heat transfer in narrow and small spaces for high heat flux requires heat transfer components with high thermal conductivity, small thickness and high reliability. In this paper, the heat transfer performance of ultra-thin vapor chambers (UTVCs) the thickness is only 0.3 mm was studied. The composite wick is made of an etching micro-structure and 300 in^-1 copper mesh. The effect of different micro-structures and liquid charging ratios for temperature distribution, the maximum temperature difference and thermal resistance of UTVCs under different heating power were analyzed, which has important guiding significance for the design and optimization of UTVCs. The experimental results show that the maximum test heating power of UTVCs can reach 5.5 W. The optimal liquid charging ratio for continuous micro-structure and non-continuous micro-structure is 60% and 80%, respectively. With the increase of heating power, there was little difference between the evaporation section temperature for the two kinds of samples, however, the temperature gradient in the length direction decreases gradually for each sample. The continuous micro-structure UTVC with a liquid charging ratio of 60% has the best thermal performance, the minimum temperature difference is 2.4 °C while the heating power is 5.5 W. Besides, the heat transfer performance test has also been carried out for placed method in a different direction.