Suspension of micrometer-sized 1,4-bis(4-methylstyryl)benzene(p-MSB)was converted into colloidal nanocrystal solution by irradiation with an femtosecond laser(800 nm,1 kHz).The prepared nanocrystals were rectangular with ca.100 nm in size.The same crystal structure as that of bulk crystals was confirmed by X-ray diffraction measurement.UV-Vis spectra and emission spectra of the nanoparticle dispersions in dichloromethane(poor solvent)were examined.The nanocrystal exhibits large quantum yield(89％).The nonlinear optical properties of the nanocrystals were further studied by Z-scan technique with femtosecond laser duration of 120 fs at a wavelength of 800 nm.The results show that the nanocrystals exhibit strong nonlinear absorption.