Abstract: As a renewable and green composite material for buildings, bamboo scrimber has the advantages of densified and uniform micro-structure, high strength, variable size and shape and high utilization rate of raw materials. Therefore, bamboo scrimber has a broad prospect in civil engineering. In order to study the quasi-brittle fracture of bamboo scrimber in the transverse direction, three-point-bending tests were carried out on single-edge-notch (SEN) bamboo scrimber samples with different sizes. Based on the boundary effect model (BEM) in fracture mechanics, a quantified relation between the micro-structure and the mechanical properties of bamboo scrimber was established by introducing an important micro-structure parameter—average grain size G. Its transverse fracture parameters including tensile strength f_t and fracture toughness K_IC were calculated by the peak loads P_max obtained from the tests. Then, the normal distribution was adopted to obtain μ_f and μ_K, the mean value of the fracture parameters as well as to cover the experimental scatters with 96% reliability, with the result as follows: f_t=μ_f=216.36 MPa, K_IC=μ_K=16.76 MPa·m^1/2. The fracture parameters can be predicted by normal-size samples in the laboratory, and the error between the predicted and tested results is only 3.17%. The fracture parameters tend to increase first and then decrease with the increase of crack length to sample height ratio.