The effects of organic-inorganic cage-type polyhedral oligomeric silsesquioxane (POSS) on the whisker formation behavior of Sn-based lead-free solders were investigated. Pure Sn and Sn3.0Ag0.5Cu (SAC305) solders were used as solder matrix, and the composite solder was fabricated with 3 wt% POSS trisilanol addition. The samples were tested under thermal cycling to accelerate whisker growth, and the temperature range varied between –40 °C and 85 °C. The surficial evolution and interfacial microstructure were observed. The results indicate that POSS would stabilize solder matrix under thermal cycling condition. Meanwhile, the strength and microhardness of solders are improved by POSS significantly, which consequently reduce the deformations in the solders caused by deformations, and inhibit whisker formation eventually.