An experimental study of heat transfer characteristics in a flattened copperwater mini-axial grooved heat pipe (AGHP) using a round AGHP with an outer diameter of 6mm and length of 210mm as the base case is reported. This AGHP was subsequently flattened to heights of 3.5， 3， 2.5 and 2mm. The temperature distribution along the axial distance, maximum heat transfer rate (MHTR), thermal resistance and evaporation heat transfer coefficient were experimentally determined for both AGHP and flattened AGHPs with different flattening height at several saturated vapor temperatures. The MHTR was found to decrease by 75% when the AGHP was flattened from a height of 6mm to 2mm. The thermal resistance was 0.08℃/W for both the round AGHP and flattened AGHP when the flattening height was greater than 2.5mm. When the AGHP was flattened to a height of 2mm, a significantly higher thermal resistance was observed. The evaporation heat transfer coefficient at the same saturated vapor temperature increased as the AGHP was flattened from 6mm to 3.5mm. However, when the AGHP was flattened in the range 3.5mm to 2mm, the evaporation heat transfer coefficient decreased.