The estimation method for strength of concrete in structures by small size cores is proposed recently. The strength of small size specimen is a little higher than that of the ordinary size core according to earlier experimental results, but it is concluded that the estimation of strength of concrete in structures is possible by small size cores. In this paper, analytical study on the estimation of strength of concrete in structures by small size cores is carried out by visco-elasto-plastic suspension element method. As the result, the strength of small size specimen is higher than that of the ordinary size specimen under ideal condition. On the other hand, the strength of small size specimen tends to decrease under eccentric loading and by using a specimen which has uneven loading surface. Earlier experimental results are compareble with the analytical ones and it is shown that accuracy is required in an experiment using by small size cores.