Histone deacetylase (HDAC) has emerged as an attractive target for the development of antitumor agents during the past decade. Previously tetrahydroisoquinoline-bearing hydroxamic acid analogue, ZYJ-25e (1), was identified and validated as a potent histone deacetylase inhibitor (HDACi) with marked in vitro and in vivo antitumor potency. In the present study, further modification of 1 led to another more potent, orally active HDACi, ZYJ-34c (4). Compared to FDA-approved drug suberoylanilide hydroxamic acid (SAHA), compound 4 exhibited higher in vivo antitumor potency in a human breast carcinoma (MDA-MB-231) xenograft model and in a mouse hepatoma-22 (H22) pulmonary metastasis model and similar in vivo antitumor potency in a human colon tumor (HCT116) xenograft model.