Carrier concentration, superconductivity, and crystal structure in a series of YBaSrCu3_xSnxO 3 samples (0 ~< x ~< 0.25) have been investigated. The X-ray diffraction analysis and structural computer simulations reveal that at low doping levels (x < 0.15) Sn preferentially occupies the Cu(1) site, but gradually goes into both the Cu(1) and Cu(2) site at higher doping levels. The rate of T c suppression caused by Sn doping is significantly slower for low dopant concentrations than for high dopant concentrations, indicating that different mechanisms operate at the Cu(1) and Cu(2) sites. The hole concentration, determined by both Hall effect and oxygen content, shows a systematic decrease with the dopant concentration. Our results suggest that one of the mechanisms of the superconductivity suppression is the reduction of the carder concentration, which dominates the Te-x relationship at low dopant concentrations. Another mechanism may be the impurity-scattering effect caused by the Sn doping at CuO 2 planes which is responsible for the dramatic suppression of T~ at high dopant concentrations. The doping behaviour in the present system and in the Sr free system are also compared.