The 5snd and 5snd Rydberg series of Sr were selectively excited from the 5s2 1So ground state by using two pulsed tunable dye lasers pumped by the harmonic of a Nd: YAG laser. Afterwards, the Rydberg atoms were photo-ionized by a third laser pulse. The energy levels and quantum defects have been measured for more than seventy Rydberg states of Sr.