This document proposes a new method to measure the credibility of sentiment polarity in tweets based on the author's background knowledge and expertise. It consists of two main components: a sentiment analyzer to classify tweet polarity and an opinion credibility calculator. The credibility calculator considers features like the author's bio, lists, topic-related tweets, retweets, friends/followers to assign an expert score. It then combines the expert score with the sentiment analysis result to provide an overall credibility measurement for the tweet polarity. An experiment on Hong Kong protest tweets aims to evaluate the accuracy of the proposed credibility calculation approach.