To analyze cardiac magnetic resonance imaging (CMR) characteristics in patients with isolated left ventricular noncompaction (IVNC) and assess its value in the diagnosis of IVNC in a Chinese adult Han population. We collected a consecutive series of 30 patients with IVNC from January 1, 2007, to December 31, 2008. During the same period, we prospectively included patients drawn from groups given a potential differential diagnosis for IVNC. All magnetic resonance images were analyzed using 17-segment model. Left ventricular ejection fraction was significantly lower for patients with DCM (16.2 ± 5.2%, P < 0.001) and higher in AR (47.6 ± 16.2%, P = 0.009), AS (54.6 ± 21.1%, P = 0.001) and HHD (62.4 ± 6.8%, P < 0.001) compared with IVNC (33.0 ± 14.1%). The two-layered structure was most frequently seen at the apical segments, followed by the mid-cavity and basal segments in patients with INVC. The anterior and lateral walls were more commonly involved in patients with IVNC. The number of noncompacted segments and end-diastolic ratio of non-compacted to compacted myocardium (NC/C ratio) was greater in patients with IVNC than in other five groups. The end-diastolic NC/C ratio of >2.5 had 96.4% sensitivity and 97.4% specificity for identifying patients with IVNC. CMR provides an accurate and reliable evaluation of the localization and extent of noncompacted myocardium at end-diastole. The end-diastolic NC/C ratio of >2.5 had high diagnostic accuracy for IVNC in a Chinese adult Han population.