A process for preparing an o-alkylated rapamycin derivative represented by the following general formula (1) is provided. The process includes the steps of reacting rapamycin with an alkyl triflate, purifying the resulting reaction product with a normal phase chromatograph and further purifying a purified product, which has been purified with the normal phase chromatograph, with a reverse phase chromatography wherein R represents an alkyl, arylalkyl, hydroxyalkyl, alkoxyalkyl, acyloxyalkyl, aminoalkyl, alkylaminoalkyl, alkoxycarbonylaminoalkyl, acylaminoalkyl, or aryl.