Chlorella, a microalga massively produced from biological fixation of CO2, was investigated to evaluate the possibility of processing it with polyethylene. Chlorella grain, a hollow aggregate sphere constituted of chlorella cells binding with each other by hydrogen bonds, exhibits a higher thermal stability than cellulose and strong resistance to cracking under mechanical forces up to 150 MPa, possibly due to the spherical structure and unique manner of assembly of chlorella cells. These characteristics of chlorella strongly indicate that it can resist deformation and thermal decomposition in a compounding process with polyethylene.