This study investigates the effectiveness of Guided Inquiry-Based Instruction (GIBI) integrated with Variation Theory in improving grade ten students’ solid geometry achievement in Debre Tabor City, Ethiopia. A quasi-experimental design involving 99 students found in three classes from three government schools assigned them randomly to three groups: Experimental Group 1 (EG1, n=30) received GIBI with Variation Theory, Experimental Group 2 (EG2, n=37) received only GIBI and the Control Group (CG, n=32) followed traditional methods. Pre- and post-tests analyzed using ANCOVA and paired t-tests revealed significant improvements, with EG1 achieving the highest scores (p = .000). Effect sizes were substantial for EG1 (Cohen's d = 1.50) and EG2 (d = 1.39) compared to CG (d = .73). The results highlight that GIBI combined with Variation Theory significantly enhances students’ solid geometry achievement, emphasizing the value of such kind of innovative teaching strategy to foster students’ achievement in similar educational contexts.