Abstract: Two parameters L_Sn and L_Gn are presented to describe the sequence distribution of substituents along CMC. The percentage of substituted anhyd- roglucose residues(i.e. substitution index SI)and the degree of substitution in substituted residues can also be obtained from L_Sn and L_Gn through simple calculations. Seven CMC samples of different degree of substitution(DS), SI and L_Sn were prepared and the properties of the drilling mud treated by them were studied. It was found that the fluid-loss of the mud decreased with increasing DS, SI and L_Sn of CMC. The salt-tolerance ofthe mud was enhanced with increasing L_Sn. Preliminary explanations are proposed for the above results.