A 5-methoxytryptophan and its derivatives are disclosed, wherein the 5-methoxytryptophan and its derivatives are represented by the following formula (I): (I) wherein R1, R2, R3, R4, R5, and n are defined in the specification. In addition, the present invention also provides novel used of the 5-methoxytryptophan and its derivatives for treating inflammatory-related disease and cancers.