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This work reports on the electrical (static and dynamic) as well as on the optical characteristics of a prototype matrix of Silicon Photomult ipliers (SiPM). The prototype matrix consists of 4×4 SiPM’s on the same substrat fabricated at FBK-irst (Trento , Italy). Each SiPM of the matrix has an area of 1 ×1mm2 and it is composed of 625 microcells connected in parallel. E ach microcell of the SiPM is a GM-APD (n +/p junction on P+ substrate) with an area of 40 ×40μm2 connected in series with its integrated polysilicon quench ing resistance. The static characteristics as breakdown voltage, quenching resistance, post-breakdo wn ark current as well as the dynamic characteristics as gain and dark count rate have been analys ed. The photon detection efficiency as a function of wavelength and operation voltage has been also estimated.