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The effect of 0.4, 0.6, and 0.8 moles of potassium addition in Y123 (YBCO) without changing the stoichiometric of YBCO through the thermal treatment synthesis method was investigated. The effect on potassium concentration addition on the crystal structure, surface morphology, and superconductivity property was determined by using X-ray diffractometer (XRD), Scanning Electron Microscopy (SEM), and AC susceptometer (ACS). From the XRD result, the crystal structure of YBCO showed potassium enhanced YBCO crystal growth in the direction of (00l) plane, and the volume fraction was increased when the potassium concentration increased, yet the orthormbicity decreased. However, no orthorhombic-to-tetragonal transition occurred as the structure was preserved in all samples. SEM result showed the grain size of Y123 was larger for the sample with potassium addition up to 6.37 µm for 0.6-mole potassium addition. This improved the linking of YBCO grains by narrowing the grain boundary, thus improving the pinning loss, Tp values up to 88.48 K. The intergranular loss peaks observed became narrow with the increasing of potassium concentration for sample 0.8 mole. From ACS analysis, the transition temperature (Tc) was reduced for samples with 0.4 moles and 0.6 moles of potassium and then increased for samples with 0.8 moles. The highest Tc was obtained in 0.8 moles followed by pure Y123 with 92.52 K and 92.34 K respectively. It shows the improvement of the Tc performance due to the well-oriented grains of the samples with a higher phase of YBCO than the pure sample. [ABSTRACT FROM AUTHOR]