Protein aggregation could be problematic as causes of diseases and hindrance in the production of useful recombinant proteins. Aggregation of mutant tryptophan synthase $alpha$-subunits was examined by treatment with urea and at high temperature. Large amorphous aggregate seemed to appear by heat treatment, while more various aggregates in size were formed by treatment with urea at low concentration. The result indicates that different aggregate in size could be formed depending on the treatment condition, suggesting different mechanisms underlying aggregation processes.