The present invention relates to a method for preparing dimethylether from methanol using a membrane reactor, more particularly to a method for preparing dimethylether from methanol using a membrane capable of carrying out a reaction and a separation at the same time while preparing dimethylether from methanol. Because water vapor generated by dehydration of methanol can be selectively removed from the catalytic reaction zone, decrease in catalytic activity can be prevented and thus life span of a catalyst can be extended. Further, reaction efficiency can be improved even at a temperature milder than the conventional one for dimethylether preparation, and also additional steps of separation and purification after completion of the reaction is no longer necessary.