Objective To evaluate the effects of sacrectomy extent on the stabil ity of lumbo-il iac fixation using single or dual il iac screw technique, and to determine which conditions require the dual il iac screw technique. Methods Nine fresh L2 to pelvic specimens were harvested from donated adult cadavers. After testing the intact state simulated by L3-5 pedicle screw fixation, sequential partial sacrectomies and L3 to il iac fixation using bilateral single il iac screw (Single) wereconducted on the same specimen as follows: in group A, under S1 partial sacrectomy and Single; in group B, under 1/2 S1 partial sacrectomy and Single; in group C, one-side (left) sacroil iac joint resection and Single; in group D, total sacrectomy and Single; and in group E, the single il iac screw was replaced by dual il iac screws based on group D. Biomechanical testing was performed on a material testing machine under 0-800 N compression and —7-7 N•m torsion loading modes for construct stiffness evaluation. Results The compressive stiffness of intact condition was (392 ± 119) N/mm, groups A, B, C, D, and E obtained 106.4% ± 9.5%, 102.7% ± 8.0%, 92.2% ± 10.1%, 72.7% ± 8.0%, and 107.7% ± 10.7% of intact condition, respectively. No significant differences were found among groups A, B, C, and the intact state (P gt; 0.05), however, the four groups showed significantly higher compressive stiffness than group D (P lt; 0.05). Although group E exhibited a comparable compressive stiffness with groups A, B, and intact state (P gt; 0.05), it displayed markedly higher compressive stiffness than groups C and D (P lt; 0.05). The torsional stiffness of intact state was (3.22 ± 1.23) N•m/deg. Groups A, B, C, D, and E acquired 105.4% ± 10.1%, 89.8% ± 12.3%, 75.9% ± 10.6%, 71.2% ± 10.2%, and 109.1% ± 16.9% of intact state, respectively. No significant differences were detected among groups A, B, E, and the intact state (P gt; 0.05). However, groups C and D showed remarkably lower torsional stiffness than groups A, E, and the intact state (P lt; 0.05). Importantly, group E offered remarkably higher torsional stiffness than group B (P lt; 0.05). Conclusion After under 1/2 S1 partial sacrectomy, single il iac screw technique could effectively restore local stabil ity; whereas it could hardly provide adequate stabil ity for further resection of one-side sacroil iac joint or total sacrectomy; in such situation, the use of dual il iac screw technique could obtain sufficient construct stabil ity. Therefore, in the surgical treatment of sacral tumor, the dual il iac screw technique should be considered for the unstable conditions of totalsacrectomy or under 1/2 S1 sacrectomy with one-side sacroil iac joint resection.