A series of novel 3-methyl-6-aryl-7-aroyl-6,7-dihydro-5H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazines were designed, synthesized and tested for their antiproliferative activity against HepG2 cell lines in vitro by the standard SRB assay and plant growth regulation activities on wheat (amonocotyledon) and radish (adicotyledon). The results indicated all the title compounds exhibited a very weak antiproliferative activity against HepG2 cell lines in vitro unexpectedly, while showed potent plant growth-regulating activities on both wheat and radish. The crystal structure of trans-4d was obtained from X-ray diffraction: C₁₈H₁₃N₄OSCl₃, M_r = 439.75, monoclinic system, space group P2₁/n, a = 5.3224(7), b = 14.3578(18), c = 24.442(3) Å, β = 94.180(2)°, V = 1862.8(4) ų, F(000) = 899, Z = 4, D_c = 1.5679 g/cm³, λ = 0.71073 Å, μ = 0.621 mm^‑1 and the final R = 0.0382 for 3274 unique reflections with 2851 observed ones (I > 2σ(I)).