Methylation of DNA is one of the important regulatory mechanisms of gene transcription. B-cell chronic lymphocytic leukemia/lymphoma 11B (BCL11B) plays a key role in the development, proliferation, differentiation, and survival of T cells. The aim of this study was to evaluate promoter methylation of BCL11B gene and its mRNA level in peripheral blood mononuclear cells (PBMCs) of ankylosing spondylitis (AS) patients in relation to healthy controls and evaluate their correlation with diseases clinical indices. Fifty AS patients and 50 healthy controls entered in this study. PBMCs were isolated from whole blood and RNA and DNA contents of leukocytes were extracted. The expression level of BCL11B gene was measured through real-time PCR assay using SYBR Green Master Mix, and PCR products of bisulfite-treated DNA were sequenced to determine the methylation level of promoter. Decreased transcript and increased promoter methylation levels of BCL11B gene were identified in AS patients compared with healthy controls. Hypermethylation of CpG3 and CpG5 was associated with increased AS risk. Promoter hypermethylation and mRNA overexpression correlated with each other but not with clinical manifestations. We identified aberrancies in expression and methylation of BCL11B gene in AS patients compared with healthy control group; however, they might not impress the clinical face of AS.