Oncoprotein18 (Op18) is a 19 kDa cytosolic phosphoprotein critical for cell growth and differentiation. Unphosphorylated Op18 associates with £\£]-tubulin heterodimer to form 2 tubulin-1 stathmin T2S complex and promotes microtubule catastrophe in interphase. Numerous cellular signals activate downstream protein kinases result in the phosphorylation of Ser16, Ser25, Ser38 and Ser 63 residues in Op18 that lowers its affinity for tubulin thereby increases the stability of microtubule and facilitates the formation of spindle during mitosis. Here, we found in addition to phosphorylation, Op18 could also be ubiquitin modified in vivo. An expression plasmid encodes for mutant EGFP-Op18-M5K protein whose potential lysine residues K42, K53, K75, K104, and K119 were mutated to arginines was generated to investigate the effect of ubiquitin modification of Op18 on the tubulin polymerization. Our results revealed a decrease of ubiquitin modification of mutant EGFP-Op18-M5K in comparison with that of wildtype EGFP-Op18. The expression of mutant but not the wildtype Op18 resulted in a significant increase of polymerized tubulin in mitotic cell implying that they might exhibit differential tubulin binding affinity. Moreover, the result of western blotting showed that the mutant Op18 detected in the mitotic cell corresponds to the phosphorylated version of Op18. In summary, these results imply the ubiquitination of Op18 might interfere with its phosphorylation and decrease its tubulin binding potential, thereby facilitates the polymerization of tubulin in mitotic cells. The in vitro tubulin polymerization assay will be performed to further confirm the above finding.