A thiolation method for modifying nanodiamonds includes steps as follows. A carboxylation step is provided, wherein the nanodiamonds are reacted with an oxidant for generating carboxyl groups on surfaces of the nanodiamonds so as to form carboxylated nanodiamonds. A hydroxylation step is provided, wherein the carboxyl groups of the carboxylated nanodiamonds are transformed into hydroxyl groups so as to transform the carboxylated nanodiamonds into hydroxylated nanodiamonds. A thiolation step is provided. In the thiolation step, the hydroxylated nanodiamonds, a sulfur source and a first acidic substance are mixed and then stirred so as to form a first mixture, the first mixture is added portion-wisely into an alkaline solution so as to form a second mixture, and the second mixture is acidified so as to form an acidified solution having a pH value ranging from 2 to 3, thus the hydroxylated nanodiamonds are transformed into thiolated nanodiamonds.