Organosilicon block copolymers were synthesized by co-hydrolysis and condensa-tion of hydroxyl-end-blocked dimethylpolysiloxanes，methyltriehorosilane，phenyltrieh-lorosilane andd dimethyldichlorosilane．The relationships between resin struetures and crack-resistant properties of the plastic were investigated by measuring mechanical loss．temperature spectrum with vibrational reed method．The resin shows optimum crack-resistant properties when R/Si=1.50．hydroxyl-endblocked dimethylpolysiloxane used is about 23％ of the total weight of the resin．The high frequency power tran．sistors encapsulated with this resin show no cracks and have very stable electrical parameters after a series of examination under extreme cvnditions．