  The pentaquark state of $\Theta^{+}(uudd\bar{s})$ has been observed to decay
with two decay modes: $\Theta^+\to n K^+$ and $\Theta^+ \to p K^0$. The decay
probability ratio of the two decay modes is studied with general symmetry
consideration of isospin, spin, and parity. We arrive at a result of the ratio
$\frac{\Gamma(\Theta^+\to nK^+)}{\Gamma(\Theta^+\to pK^0)}
=\frac{(\alpha-\beta)^2}{(\alpha+\beta)^2}(\frac{k_1}{k_2})^{2L+1}$, which is
valid for the $\Theta^+$ state to be a pure isoscalar or isovector state, or an
isoscalar and isovector mixing state, or an isotensor state with mixture of
isoscalar and isovector components with coefficients $\alpha$ and $\beta$. The
dependence on spin and parity of the pentaquark $\Theta^+$ state is found to be
small due to small difference between the center of mass decay momenta $k_1$
and $k_2$ of the two decay modes. We also provide an analysis on the constraint
of the isospin of $\Theta^+$ from the absence of a peak in the $pK^+$ invariant
mass distribution in the $\gamma p \to pK^+K^-$ process. Future experimental
results about the decay probability ratio may provide information about the
properties of the pentaquark $\Theta^+$ state.
