Abstract: Three kinds of sand samples with different particle sizes are selected to carry out microbially induced calcium carbonate (MICP) grouting tests. The effects of particle sizes on sand strength are studied by grouting for 8，10, and 12 times，respectively，in combination with the data such as bacterial adsorption rate, effluent Ca2+ concentration, permeability coefficient, calcium carbonate content, pore structures, and final reinforcement effects. The results show that there is a certain relationship between bacterial adsorption rate and particle size，that is, the larger the particle size, the less bacterial adsorbance. In the meantime, particle size will affect the reinforcement process of samples and the development of pore structures. A sample with a smaller particle size can retain more nutrients. During the reinforcement process, sand samples with small particle sizes is easy to form blockage in the upper part of the sample due to small pores and low permeability coefficients, leading to uneven reinforcement effects. Samples of large particle sizes，large pores，weak water holding capacity，low calcium carbonate content，are mainly deposited in the lower part，leading to poor reinforcement effects.

Key words: particle sizes, grouting test, pore structure, reinforcement effect, calcium carbonate content