Our recent research of various Th-bearing systems revealed bulk superconductivity in ThIrSi3, ThIr2Si2 and ThCoSi. The critical temperatures determined from the resistivity and magnetization data are Tc = 1.75 K, 1.9 K and 3 K, respectively. The compounds crystalize in different crystal structures, which lack an inversion center. ThIrSi3 adopts the BaNiSn3-type unit cell (s.g. I4mm), ThIr2Si2 has the CaBe2Ge2-type structure (s.g. I4mm), and ThCoSi forms with the LaPtSi-type unit cell (s.g. I41md). The bulk properties measurements indicated significant differences in the thermodynamic parameters characterizing their superconducting states. In particular, the upper critical field was found to range from 0.03 T in ThIrSi3 to over 4.5 T in ThCoSi. The aim of this proposal is to made a comparative study of the nature of superconductivity in Th-based superconductors by means of muon-SR study.