The Thulium fiber laser (TFL) generates a focused beam, which can be transmitted to laser fibers with small core diameters and may facilitate in situ lower pole lithotripsy. This study compares lithotripsy performance of the 150 and 200 µm TFL in a lower pole benchtop kidney model. Using a 3D model printed from an actual kidney, in situ laser lithotripsy was performed on 1 cm lower pole BegoStones (CaOx monohydrate consistency) using four different settings (all 20W) and two fiber sizes (150 and 200 µm). Procedure time, laser time, total pulse energy, and fiber stripping were compared between the two fibers using an ANOVA or independent t-test. The 150 µm fiber at 0.2J x 100Hz had the shortest lasing and procedure time (17.3 and 18.5 min.) and lowest total pulse energy (20.75 kJ) compared to other study arms (p<0.001). Overall procedure time, lasing time and total pulse energy were significantly different between the 8 settings (p<0.001 for all). At higher frequency (100 and 200 Hz), lasing time was significantly faster compared to 20 and 50Hz (19.9 vs. 27.3 min. p<0.001). Furthermore, the average total procedure time was shorter with 150 µm compared to 200 µm regardless of settings (23.2 vs. 29.8 min.; p<0.001). The 150 µm fiber results in shorter procedure and lasing time at lower total energy levels during lower pole in situ lithotripsy. Overall, the fastest setting was 0.2J and 100Hz with the 150 µm fiber. Smaller laser fibers can potentially allow more efficient in situ laser lithotripsy with better irrigation and visibility at higher deflection angles.