The post-translational modification of proteasome has been investigated to determine the relation between function and the post-translational modification in the proteasome. The 26S proteasome is a large protein complex with proteolytic activity, consisting of 20S proteasome and 19S regulatory particle. In yeast, there are at least three N-acetyltransferases (NAT), NAT1, MAK3 and NAT3. Among 14 subunits of the 20S proteasome, the α1, α2, α3, α4, α7 and β3 subunits have been found to be N-acetylated with NAT1, the α5 and α6 subunits with MAK3, and the β4 subunit with NAT3. It was shown that chymotrypsin-like activity is slightly higher in the NAT1 deletion mutant than the normal strain, suggesting that N-acetylation be related to the proteolytic activity of 20S proteasome. The N-acetylation of 20S proteasome subunits in plants have been studied. In addition to the N-acetylated subunits in yeast, the β6 subunit has been deduced to be N-acetylated in plants. Similarly, N-terminal modification of the 19S regulatory particle has been investigated in yeast. Among 17 subunits identified in the 19S regulatory particle, 13 have been found to be N-terminally modified and at least 11 to be N-acetylated. On the other hand, phosphorylation is well-known post-translational modification in proteins. The phosphorylated subunits have been identified three in the yeast 20S proteasome and two in the mammal 19S regulatory particle. The relation between the proteolytic activity or assembly and phosphorylation of proteasome has been studied, suggesting that the post-translational modification of proteasome is important for its functions.