  In this paper we give a proof of the equivalence between the RNS and Pure
Spinor formalism for Type I tree level amplitudes involving up to four
fermions. This result have been obtained previously for amplitudes involving
only closed or open amplitudes and here we extend it to include Type I
amplitudes. For this we first prove the cyclic symmetry of Type I superstring
in a way that it is also valid for the Pure Spinor formalism. The technique
used is applied to simplify the calculation of the tree level three point
amplitude previously computed. As a byproduct, we are able to calculate the
gauge anomaly amplitude of type I Superstrings in this formalism.
