It was discovered that newly synthesized RubisCO large subunits could bound to another high molecular weight protein that occurs as a complex, when the intact chloroplast was incubated with S-Met. The addition of ATP caused dissociation of the complex. The released large subunits then assembled into RubisCO. This high molecular weight protein is called RubisCO subunit binding protein, the protein plays an important role in the assembly of RubisCO. The purification of binding protein was directly started with extract of leaves. The purified binding protein was judged by ND-PAGE, SDS-PAGE and agarose double immunodiffusion. Finally it was obtained that 90% of then protein was the binding protein. It was measured that the surface SH groups of the purified binding protein were 13 ± 1 and the total SH groups were 37 ± 1. The measured and calculated results of the intrinsic region CD spectra showed that there was 39% a-helix and a typical character of a-helix spectra in the binding protein. The binding protein was not dissociated and denatured by heating at 50 ℃. This result shows that the binding protein is a stable protein against heat. The addition of ATP to extract of leaves, which had lower concentration of protein, caused dissociation of the binding protein dodecamer, and ADP did not cause its dissociation of the binding protein. Antibodies raised against the purified pea binding protein had highly specific cross-reaction with the binding protein, but it did not cross-react with the pea RubisCO, nor with the RubisCO large and small subunit from tobacco.