The recognition phase of an Optical Character Recognition (OCR) system produces a ranked list of candidate characters, among which the top one is usally taken as recognition result without taking context into account. Recognition error occurs if the correct character is not at the top, which is mostly due to shape similarity between characters.In this paper we propose to use character trigram, which means that two previous characters are taken into account when choosing the character from the candidate list as recognition result for Khmer OCR.A text corpus of about 300 Mbytes is used to compute character trigrams. Using these trigrams, we test our approach on about 3000 characters. The result shows that this approach can correct about 30% of recognition errors.