Well-dispersed uniform SrMoO_4 nanocrystals were synthesized by thermal decomposition of a metalorganic salt in the organic solvent under different temperatures(80, 100, 120, 140, and 160?C). The smallest diameter of these SrMoO_4 nanocrystals is only about 2 nm, which is regarded as the smallest values to date. The UV–vis absorbance spectra present that the larger absorption of our samples is mainly distributed in the visible light region and UV light region. The lowest energy gap is found to be 2.71 e V.Such a small gap is ascribed to the introduction of intermediate energy levels, which are due to the surface defects with decreasing the size of nanostrcutrues. The photoluminescence measurement suggests that all these samples exhibit a board and strong emission band in the range from 500 to 700 nm. Through the deconvolution of the photoluminescence spectra, the emission profiles are found to be associated with three various components(green, yellow, and red). Moreover, the photodegration of methyl blue over our SrMoO_4 samples reaches nearly 100% in 120 min. Such a high photodegration may be related to the following aspects. One is related to the size and morphology. Larger surface area leads to more absorption of methyl blue, and the small size nanoparticles lead to the efficient separation of these photogenerated electron-hole pairs. The other is related to the narrow band gap. The small gap is beneficial to more electrons to be excited from the valence band to the conduction band, and eventually more electron-hole pairs are created. Our investigations clearly suggest that thermal decomposition of one metal-organic salt in organic solvent will be a good choice to synthesize the nanoparticles with small size and uniform distribution. Our results also indicate that these SrMoO_4 nanoparticles possibly have great potential utilities in photocatalysts. The smallest diameter of these SrMoO 4 nanocrystals is only about 2 nm, which is considered as smallest value to date. The UV absorption of the spectra of that larger absorption of our samples is mainly distributed in the visible light region and UV light region. The lowest energy gap is found to be 2.71 e V.Such a small gap is ascribed to the introduction of intermediate energy levels, which are due to the surface defects with decreasing the size of nanostrcutrues. The photoluminescence measurement suggests that all these samples exhibit a board and strong emission band in the range from 500 to 700 nm . Through the deconvolution of the photoluminescence spectra, the emission profiles are found to be associated with three various components (green, yellow, and red). Moreover, the photodeg such as methyl blue over our SrMoO_4 samples reaches nearly 100% in 120 min. Such a high photodegration may be related to the following aspects. One is related to the size and morphology. Larger surface area leads to more absorption of methyl blue, and the small size nanoparticles lead to the efficient separation of these photogenerated electron-hole pairs. The other is related to the narrow band gap. The other is related to more electrons to be excited from the valence band to the conduction band, and eventually more electron -hole pairs are created. Our investigations clearly suggest that thermal decomposition of one metal-organic salt in organic solvent will be a good choice to synthesize the nanoparticles with small size and uniform distribution. Our results also indicate that these SrMoO_4 nanoparticles have great potential utilities in photocatalysts.