Histone deacetylase (HDAC) is enzymes, which inhibits histone deacetylase by removing scetyl group from n-acetyl lysine amino acid. Histone deacetylase inhibitors are emerging as potential application in diagnosis, treatment and prognosis of cancer. It is also considered as a treatment option for hematological malignancies and plays important role in epigenetic or non-epigenetic regulation. The histone deacetylase inhibitors accumulates acetylated nuclear histone in tumors and normal tissues. Thus, the inhabitation results in acetylation transcription factors such as GATA-1, p53 and estrogen receptor-alpha. These enzymes are found in fungi, bacteria, animals and plants. There are around 18 human histone deacetylase known, which are classified into four classes based on their structures. Class I HDAC1, HDAC2, HDAC3, and HDAC8. Class II includes HDAC4, HDAC5, HDAC7, and HDAC9. Class III includes HDAC6 and HDAC10, whereas Class IV includes HDAC11.