  We have realized a dual-species Bose-Einstein condensate (BEC) of $^{23}$Na
and $^{87}$Rb atoms and observed their immiscibility. Because of the favorable
background intra- and inter-species scattering lengths, stable condensates can
be obtained via efficient evaporative cooling and sympathetic cooling without
the need for fine tuning of the interactions. Our system thus provides a clean
platform for studying inter-species interactions driven effects in superfluid
mixtures. With a Feshbach resonance, we have successfully created double BECs
with largely tunable inter-species interactions and studied the
miscible-immiscible phase transition.
