In this paper theoretical and experimental studies are conducted on three kinds of micro-diameter heat pipes (MDHPs， their outer diameter is 2 mm) with composite wick． The composite wicks consist of copper powdermesh (SMCP)，copper foam-mesh (SMCF) and mesh-mesh (MSM)． By referring to the capillary limit theory， the heat transfer power of the three kinds of MDHPs is analyzed and at the same time，maximum heat transfer capability under different filling ratios， the axial temperature distribution and the evaporation and condensation thermal resistances are analyzed． The results indicate that the maximum heat transfer capabilities of the three are 7W，with the corresponding optimum filling ratios of SMCP，SMCF and MSM being 110%，95% and 90% respectively， which are close to the theoretical values of capillary limit． The axial temperature differences of MDHPs increase with the heating power，and the maximum values are 4. 22，4. 20 and 4. 90 ℃ respectively． With the increase of heating power， the evaporation thermal resistance increases gradually． The change of condensation thermal resistance is small under low filling ratio． With high filling ratio，condensation thermal resistance of SMCP heat pipe fluctuates greatly，while condensation thermal resistances of SMCF and MSM heat pipes are relatively stable． When the filling ratios of the three kinds of MDHPs and their respective optimum filling ratios and heat transfer performance reach 7W， their evaporation resistances of MDHPs are 0. 437，0. 493 and 0. 591℃ /W，and their condensing heat resistances are 0. 167，0. 106 and 0. 110℃ /W，respectively．