Potassium titanyl phosphate (KTP) nanoparticles have garnered significant attention in various applications due to their unique optical and electrical properties. In this study, KTP nanoparticles were synthesized using the hydrothermal method, and the effects of oxalic acid as a capping agent and its different ratios on the physical and structural properties of these nanoparticles were investigated.XRD results indicated that all samples formed the orthorhombic phase of KTP. The average grain size was calculated using the peak width in the X-ray diffraction spectrum and the Williamson-Hall and Scherrer equations. The use of oxalic acid as a capping agent led to a reduction in grain size, although a sudden increase in grain size was observed at a 2:1 ratio. FESEM images showed that, in the absence of a capping agent, the particles formed agglomerates and were irregular in shape, whereas using oxalic acid at a 1:1 ratio resulted in rod-like particles with smoother surfaces. Increasing the ratio of oxalic acid to 3:1 led to the formation of more complex and well-ordered structures.The results of this study demonstrate that using oxalic acid as a capping agent can help better control the size and shape of the particles, and increasing its ratio results in more complex and well-ordered structures. These findings can be beneficial in improving the synthesis processes of KTP nanoparticles and their various applications in optical and electronic industries.