The effects of thickness and surface grinding condition on the fracture strength of Si wafer with a thickness under $100{mu}m$ were investigated. Fracture strength was measured by ball breaker test for about 330 dies (size: $4mm{ imes}4mm$) per each wafer. For statistical analysis of the fracture strength, scale factor was determined from Weibull plot. Ball breaker fracture strength was observed to increase with decreasing thickness of silicon die. For the silicon dies of different surface conditions, ball breaker fracture strength was high in the order of polished, ground (#4800), and ground (#320 grit) specimen. Probabilistic fracture strength (i.e., scale factor) increased with decreasing surface roughness of silicon die.