  We study the deformation property of $\Lambda$ hypernuclei using the
relativistic mean field (RMF) method. We find that $^{29}_{\Lambda}$Si and
$^{13}_{\Lambda}$C hypernuclei have spherical shape as a consequence of the
additional $\Lambda$ particle, whereas the corresponding core nuclei, $^{28}$Si
and $^{12}$C, are oblately deformed. Most of other hypernuclei have a similar
deformation parameter to the core nucleus, in accordance with the previous
study with the non-relativistic Skyrme-Hartree-Fock method. We discuss the
sensitivity of our results to the choice of pairing interaction and to the
parameter set of the RMF Lagrangian.
