Modification of enzymes can enhance their activities in organic solvents. Horseradish peroxidase (HRP) was modified with methoxy-PEG 5000 which was linked onto HRP peptidal free aminos or onto the aminos introduced through the oxidation of sugar side chains of HRP. It was found the enzyme with PEG linked to peptide chain expressed a nearly same activity as the native HRP in aqueous system. However, PEG modified HRP by oxidation of carbohydrate chains remained only about one third activity of the native HRP. The same results were observed in reversed micelles and in dioxane of less than 30%. Apparently, both PEG modified HRP had the higher activities in high concentration of dioxane. Especially, the activities of the modified HRP in toluene were both enhanced up to nearly 3 fold. When HRP was modified with PEG through the amino groups of peptide chain, it was more stable than the native enzyme in water, dioxane and toluene. However, the stability of PEG-modified HRP by carbohydrate chains was higher in water and lower in both dioxane and toluene in comparison with native HRP.