  We report here the first observation of directed flow ($v_1$) of the
hypernuclei $^3_{\Lambda}$H and $^4_{\Lambda}$H in mid-central Au+Au collisions
at $\sqrt{s_{\rm NN}}$ = 3 GeV at RHIC. These data are taken as part of the
beam energy scan program carried out by the STAR experiment. From 165 $\times$
10$^{6}$ events in 5%-40% centrality, about 8400 $^3_{\Lambda}$H and 5200
$^4_{\Lambda}$H candidates are reconstructed through two- and three-body decay
channels. We observe that these hypernuclei exhibit significant directed flow.
Comparing to that of light nuclei, it is found that the midrapidity $v_1$
slopes of $^3_{\Lambda}$H and $^4_{\Lambda}$H follow baryon number scaling,
implying that the coalescence is the dominant mechanism for these hypernuclei
production in such collisions.
