Specific small amounts of amino acids caused agglomeration of L-valine (L-Val) crystals during evaporative crystallization from aqueous solutions. The agglomeration of L-Val occurred only under acidic condition when guest amino acids satisfied several conditions. Only L-form amino acids that have carboxylic acid groups and sufficiently long alkyl chain in the side-chains could induce agglomeration of L-Val. The length of alkyl chain in the side-chains controls the degree of agglomeration. Data indicated only 0.5wt% of L-2-aminoadipic acid, which has a similar chemical structure to L-glutamic acid (L-Glu), produced the large agglomerates >1000m. The particle size was approximate to 500m when using the same amount of L-Glu. Based on the results from previous tests and this paper, the whole mechanism for the L-Val agglomeration in the presence of specific guest amino acids has been revealed.