Disclosed herein are a polysaccharide derivative obtainable by substituting a part or the whole of hydrogen atoms of hydroxyl groups in a polysaccharide or a derivative thereof by the following substituents (A) and (B):(A) a hydrophobic group containing a C.sub.10 -C.sub.40 alkyl, alkenyl or acyl group which may be substituted by an --OH group or into which an oxycarbonyl group (--COO-- or --OCO--) may be introduced; and(B) a C.sub.1 -C.sub.5 sulfoalkyl group which may be substituted by an --OH group, or a salt thereof, wherein average degrees of substitution by the substituents (A) and (B) per monosaccharide residue are 0.001-1.0 and 0.01-2.0, respectively; a preparation process thereof; and use of the polysaccharide derivative for a thickener for cosmetic compositions.