The strength properties of cemented tailings backfill (CTB: an engineered mix of processing tailings, hydraulic binder and mixing water) is strongly affected by its structural integrity. However, our understanding of the structural integrity of fiber reinforced CTB samples as well as its evolution with time is limited. Thus, a comprehensive laboratory investigation is conducted to study the effect of fiber length on strength properties of CTB prepared with different specimen sizes. The unconfined compressive strength (UCS) tests are conducted on CTB samples prepared with four different lengths (0, 6, 12 and 18 mm) and sizes (cubic mold sizes: 40, 70.7 and 100 mm, accompanied with cylindrical mold sizes: 50 x 100 mm) of polypropylene fiber to study their strengths. Experimental results have shown that the peak and end strains of cubic CTB samples are larger than those of cylindrical CTB ones, and UCS of C-40 CTB is higher than that of the cylinder U50. At this time, the size effect and geometry are the main reasons for the differences observed in the strength properties of the above two backfilling bodies. The UCS of CTB samples decreases with increasing the volume ratio when the fiber length varies from 0 mm to 6 mm. When the fiber length varies from 12 mm to 18 mm, the size effect of fiber-reinforced CTB samples is not obvious, and the fiber length becomes the key to influence the coupling curve of two factors. In addition, the longer the fiber length does not necessarily lead to the higher UCS of CTB samples, but the integrity of the sample under the same external load is better. The findings of this study will help to better evaluate and forecast the UCS behavior of CTB samples under various fiber conditions. (C) 2020 Elsevier Ltd. All rights reserved.