Abstract: In order to study the mechanism of synthesis and degradation about Rubisco, (NH₄)₂SO₄ which was marked by ¹⁵N was applied during earing period to delay the falg leaf ageing process, meanwhile the content and ¹⁵N abundance of Rubisco, LSU, SSU, etc. were determined. The result showed that the content and the ¹⁵N abundance of Rubisco, LSU, SSU were at high level during the RAPD (Relative active photosynthetic duration) of wheat flag leaf, which indicated that Rubisco was at a continuous process of turnover about synthesis and degredation. Rubisco was resynthetized still, while Rubisco was hydrolyzed. During the RPD (rapid photosynthetic duration) of wheat flag leaf, ¹⁵N abundance of Rubisco, LSU and SSU were at a lower level, and had little changes. All these indicated that there was little Rubisco resynthesized during RPD. The ¹⁵N abundance of soil was declined, and this indicated that plant possessed a powerful ability in absorbing nitrogen from soil during the later upgrowth period. So it was very important for a higher wheat yield to apply nitrogen fertilizer in earing period.