We examined morphometric properties of alluvial fans and their source basins in Taiwan. Although studies elsewhere have found a decrease in alluvial-fan slope and basin slope with increasing basin area, the relatively large fans and source basins along the backbone of the Taiwan Central Range tend to have constant mean slopes of approximately 0.9° and 32°, respectively, with basin areas varying over the range 80–2900 km2. Smaller fans and source basins, mostly in lower areas away from the backbone of the range, have more varied slopes. The existence of the constant-slope fans and basins may reflect highly organized hillslope-channel systems that have formed under very rapid uplift and erosion. The systems are dominated by steep V-shaped valleys with similar hillslope angles and consistently high-gradient channels that enable efficient sediment flushing. This leads to similar ratios of sediment discharge to runoff and similar sediment grain sizes irrespective of basin area, resulting in alluvial fans with similar slopes.