The tensile strength of an oil well cementing composition is one of the most important values that is used in modelling programs to assess the risk of losing cement sheath integrity based on the modified Mohr-Coulomb failure criteria. The tensile strength of a cement sheath is governed by its composition and resulting microstructure which relates to the severity of the defects and, hence, it can be analyzed using Weibull statistics. The aim of this study is to apply the Weibull statistics as a tool to determine the characteristic tensile strength (TS0) of a reference Class G cement slurry and different slurries reinforced with several inorganic micro-fibers. The materials investigated included different inorganic micro-fibers in the system Ca–Si–Al and a muscovite-type additive. Direct tensile tests were performed using a classical dog-bone configuration. In addition, modified direct tensile tests using a new dog-bone configuration and an indirect tensile test (Brazilian test) were carried out and compared. It was found that the Weibull modulus and the characteristic tensile strength depend on the amount and size of micro-fiber. Finally, it is concluded that Weibull statistics can be applied to obtain greater confidence and more reliable tensile strength values for use in risk analysis related with cement sheath long-term integrity.