The aim of the present study was to establish T1 variation (T1v) thresholds for duplicated measurements of regional T1 values in left ventricle (LV) using magnetic resonance imaging (MRI). Eighteen healthy volunteers were recruited to undergo two consecutive cardiac MRI scans using modified Look-Locker Inversion recovery (MOLLI) with two spatial resolutions on different days to repeat T1 measurements on LV. The absolute differences (d) and standard deviations (SDs) of regional T1 values were acquired with the two scans and two readers. T1v threshold (mean difference + 2SD), intra-class correlation coefficient (ICC) and coefficient of variation (CoV) were calculated. T1 mapping using the MOLLI sequence (with multiple spatial resolutions) was successfully performed in all 18 volunteers twice. On a per-slice basis, ICCs for intra-observer, inter-observer, inter-resolution and inter-study T1v were 0.988, 0.899, 0.763 and 0.6. CoVs were 0.72, 2.39, 3.90 and 4.28%. T1v thresholds were 22, 66, 118 and 120 ms. On a per-segment basis, ICCs for intra-observer, inter-observer, inter-resolution and inter-study T1v were 0.974, 0.859, 0.711 and 0.594. CoVs were 1.09, 3.36, 4.69 and 5.01%. T1v thresholds were 33, 94, 140 and 144 ms. Those thresholds may be useful for discriminating disease-initiated T1v from random errors of T1 measurements.