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High-quality single-walled carbon nanotubes (SWNTs) with tunable diameters were synthesized by an improved H-2/CH4-based floating catalyst method. Transmission electron microscopy observations and Raman results demonstrated the overall quality of the as-synthesized samples with finely tailored large diameters at 1.28, 1.62, 1.72, 1.91, and 2.13 nm, depending on the experimental conditions. In addition, Raman analysis revealed that the abundance of specific (n, m) SWNTs could be selectively enriched simultaneously along with the diameter modulation. It was found that the selective etching effects of high hydrogen flow stabilized the decomposition of ultralow CH4 flow and considerably suppressed the deposition of amorphous carbon and small nanotubes, leading to very pure samples with high structural homogeneity suitable for further applications in practical electronic systems.