Water suspension of micrometer-sized vanadyl phthalocyanine (VOPc) crystals was converted into VOPc colloidal solution by irradiation with an excimer laser (351 nm, 30 ns, 5 Hz). VOPc nanoparticles with nearly equilateral triangular and hexagonal shapes were obtained from the colloidal solution, and the mean width and height are 60 and 19 nm for triangular nanoparticles, respectively, and 49 and 17 nm for hexagonal ones, respectively. Laser fluence dependence was examined and laser ablation of the microcrystals in water is considered to be responsible for the nanoparticle formation. A threshold laser fluence of the conversion was determined to be similar to20 mJ/cm(2). Nanoparticle formation processes and their phase transition during standing were analyzed by electronic absorption spectroscopy.