The effect of background pressure was investigated in silicon deposition by plasma jet CVD, a recently developed CVD method to deposit silicon at a deposition rate over 1 mu m/s. The deposition rate, deposition structure, and crystallinity of the deposited silicon were examined at pressures between 133 and 800 Pa, and it was found that the deposition rate increased with pressure increase. Drastic changes in the deposition rate and crystallinity were observed between 267 and 400 Pa. Part of the gas flow was considered to be transitional flow at 267 Pa and below. Knudsen number was calculated to estimate gas flow in the deposition chamber, and the flow was estimated to be transitional at lower background pressure conditions. Mass transfer around substrates was suppressed by decrease of the gas velocity due to the change of flow. Concentration of deposition precursors was estimated by calculation using CHEMKIN-PRO software, and the concentration of SiH2 was found to increase with decrease in the background pressure. Variation of crystallinity was explained by variations in the gas velocity of the main jet and precursor concentrations. Thus, background pressure was an important factor in the use of plasma jet CVD.