Nano-micron-sized spheroidal aggregates of FeS2 has been found from the Laozuoshan gold deposit, Heilongjiang Province, NE China. There are diversified morphology, some as large or small girdles, and some as paragenesis of aggregates through cementation. Most of such aggregates distribute at the edge of pyrrhotite before metasomatized by arsenopyrite. Through a systematic observation on the micro-structure, grain size and components of nano-micron-sized spheroidal aggregates of FeS2 by means of Field Emission Scanning Electron Microscopy (FE-SEM), Electron Probe Micro Analysis (EPMA) and in situ micro X-ray Diffraction (XRD), etc. We found such aggregates are composed of nano-micron-sized grains gathered together, part of which have an euhedral structure, with a crystalline grain size of 50 nm-<1 mu m. The aggregates of FeS2 are composed of pyrite and marcasite, and the crystal face diffraction peak intensities of pyrite (200) and (311) decline remarkably from the center to the edge, and the crystal face diffraction peak intensities of marcasite (110) and (311) in the center are lower than those at the edge. From the center to the edge, the content of Fe in nano-micron-sized spheroidal aggregates changes slightly, but the S content declines remarkably at the edge, and the content of As is low in the center but grows remarkably at the edge. In addition, there are low-temperature minerals such as chalcedony and carbonates (like calcite) filled along the edge of or in the fissures among spheroidal aggregates of FeS2. Based on the above study in combination of the fluid inclusion characteristics of the mineral deposit, we suggest that the spheroidal aggregates of FeS2 are formed under the drive of free energy on the surface of nano-sized FeS2 crystals, which nucleates rapidly in a static fluid environment due to fluid getting supersaturated. The spheroidal aggregates of FeS2 indicate that under low temperature conditions, the pH value of fluid changes from medium acidity to acidity throughout the crystallization process.