Objective: The ultimate goal of this study is one of the discourse fields analysis, i.e. Grice's cooperative principles.

Materials & Methods: In this comparative and case - control study, two groups of normal individuals and schizophrenic patients were selected by simple method from sample of convenience and interviewed based on the spontaneous part of Persian Aphasia Test. All the answers were analyzed with respect to the percent rate of errors in the 4 "cooperative maxims", and the errors were collected. Data were analyzed by Independent T Test, ANOVA, Kruskal Wallis and Mann Whitney.

Results: There were significant differences between two groups in respect to all the Grice’s cooperative principles (P<0.001), maxim of quantity (P<0.001), maxim of relevance (P<0.001) and maxim of manner (P=0.02), but in maxim of quality there was no significant differeance (P=0.07). The mean percent of errors in the course of illness, education and age varieties were not significant.

Conclusion: Schizophrenic patients have incoherence speech and this problem exists in the discourse field and social interactions, thus the patients frequently fail in observing the Gricean cooperative principles.