Metropolitan Rapid Transit (MRT) (recommend)
The Metropolitan Rapid Transit, or MRT, is a rapid transit system serving the Bangkok Metropolitan Area and the Sam Yan MRT Station is right next to the Mandarin Hotel. Head towards Exit 1 (Hua Lumpong Temple) to the Rama IV Road. Walk straight on Rama IV Road past the Second Church Samyan and reach the Mandarin Hotel.
More detail: (http://www.bangkokmetro.co.th)