BACKGROUND: In low flow oxygen therapy, FIO2 is difficult to measure in spontaneously breathing patients due to room air dilution and dead space rebreathing, especial in impairments respiratory mechanics. This study determined oxygen concentrations with different tidal volumes and respiratory rates among different lung mechanics and provided equations to estimate oxygen concentrations during standard nasal cannula oxygen therapy. METHODS: Two Training & Test Lung models were used in this study. One simulated spontaneous breathing, whereas the other included an expiratory gas modification bellow. Three lung mechanics [normal (R5/C60), restrictive (R20/C80) and obstructive (R5/C40)] were designed, and spontaneous breathing settings for different tidal volumes(VT) and respiratory rates(f) were simulated by the mechanical ventilator. The nasal cannula used flows of 1, 3 and 5L/min; peak inspired oxygen concentration (FO2 insp.) and pre-inspired oxygen concentration (FO2 pre-insp.) were measured. RESULTS: Increased VT caused a decreased FO2 insp. and FO2 pre-insp., except at 1L/min oxygen flow with a high f (30breaths/min). Multiple regression analysis showed oxygen flow rate, VT and f as the most important factors in predicting oxygen delivery during nasal cannula therapy. Therefore, we provided equations to predict oxygen concentration for managing patients with acute and chronic lung diseases. CONCLUSIONS: Our study suggested that under low-flow nasal cannula therapy, various lung mechanics and respiratory patterns in the normal, restrictive and obstructive lung models will affect the oxygen concentration.