Class B gene APETALA3 (AP3) plays a key role in the development of petals and stamens. Here, we investigated the expression pattern of PAP3 gene (genbank accession number: HM104635) in the buds of cytoplasmic male sterility line 121A and its near-isogenic restorer line 121C at four developmental stages and analyzed the possible association between Class B genes and cytoplasmic male sterility of pepper. Semi-quantitative PCR and quantitative real-time RT-PCR (qRT-PCR) as well as RNA in situ hybridization showed increased expression of PAP3 at late phase of anther development and its higher expression in restorer line compared with sterility line indicating PAP3's role at late developmental stage of anther and suppressed expression in sterility line. RNA in situ hybridization showed Class B gene features: high abundance in stamen and petal; lower expression in pistil; no expression in sepal. Results of transient expression in onion epidermal cells also showed PAP3 localized in the nucleus, which is consistent with the expression pattern of transcription factors of MADS-box gene family.