Objective To explore the clinical phenotype and genotype of suspected restrictive cardiomyopathy caused by TNNI 3 gene mutation. Method The clinical data from a child suspected of restrictive cardiomyopathy were retrospectively analyzed. Results A 4 -year- 8 -month-old girl was admitted to hospital due to cough, fever and abnormal electrocardiogram. The electrocardiogram showed heavy load in both atria and high left ventricle voltage. Echocardiography showed whole heart enlargement with weakened motion, cardiac insufficiency, pericardial effusion (small amount), mitral regurgitation (moderate reflux) and pulmonary hypertension. Two months before admission, the child had a history of cyanosis when he went upstairs. Genetic test indicated that the child had a heterozygous mutation of c. 575 G>A which was a new mutation in TNNI 3 gene, and her parents did not carry the mutation. This mutation caused the arginine at position 192 of the TNNI 3 to be replaced by histidine. Multi-species comparison found that this site was highly conserved. Various prediction softwares suggested that the mutation was deleterious. Conclusion? The heterozygous mutation of c. 575 G>A (p.Arg 192 His) in TNNI 3 gene was likely to be the pathogenic mutation of the child.