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The pentaquark state of Θ+(uudd&smacr;) has been observed to decay with two decay modes: Θ+ → nK+ and Θ+ → pK0. 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 [This symbol cannot be presented in ASCII format]2L+1, which is valid for the Θ+ 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 α and β. The dependence on spin and parity of the pentaquark Θ+ state is found to be small due to small difference between the center-of-mass decay momenta k1 and k2 of the two decay modes. We also provide an analysis on the constraint of the isospin of Θ+ from the absence of a peak in the pK+ invariant mass distribution in the γp → pK+K- process. Future experimental results about the decay probability ratio may provide information about the properties of the pentaquark Θ+ state. [ABSTRACT FROM AUTHOR]