Intrinsic plagiarism detection deals with the task of finding plagiarized sections of text documents without using a reference corpus. This paper describes a novel approach to this task by processing and analyzing the grammar of a suspicious document. The main idea is to split a text into single sentences and to calculate grammar trees. To find suspicious sentences, these grammar trees are compared in a distance matrix by using the pq-gram-distance, an alternative for the tree edit distance. Finally, significantly different sentences regarding their grammar and with respect to the Gaussian normal distribution are marked as suspicious.