Raman spectra of single-walled carbon nanotubes (SWNTs) with diameter distribution of 0.6–1.6 nm have been studied under high pressure. We focused on the dependence of radial R-band frequency (associated with the radial breathing mode, RBM) on pressure, dωRBM/dP, and studied the effect of experimental factors on the structural changes of nanotubes under pressure. We found that the nanotube diameter and the pressure transmission medium (PTM) strongly affect the dωRBM/dP dependence of a nanotube in the experiments with PTMs, while the nanotube diameter and the intertube interactions between the tube walls play a dominant role on the upshift of RBMs for nanotubes without a PTM. These results are in line with the theoretical predictions that PTMs have a strong effect on the pressure-induced shift in the R-bands of SWNTs. In addition, we found that the collapse of nanotubes and their reversibility depend not only on diameter and pressure environment but also on the sample quality. This could explain that there have been different results reported by different groups even for nanotubes with the same diameter distribution.