The probabilistic theory of Type 1 size effect can be derived from fracture nano-mechanics. Kramer's transition rate theory shows that, on the nano-scale, the far-left tail of the probability distribution of nano-scale strength s is a power law of the type s 2 . Analysis of the multiscale transition to the material macro-scale then shows that the RVE strength distribution is Gaussian but with a Weibull (or power-law) left tail whose exponent m is much larger than 2 and is grafted roughly at the probability of about 0.001.