This research studies the impact of Trump’s twitter behaviour on US public companies by examining the relationbetween Trump’s tweets and stock returns of the companies he mentions. Twitter data is collected from the day that Trump became president, on 1 January 2017 up toMarch 2019. S&P500 data isused for the stock prices. The tweets are categorised into good, bad and neutral tweets. The resultsshowthatvolatility increases due to Trump’s tweets and that thestock price of positively(negatively) mentionedfirms increase (decrease) after the tweet initially, but the abnormality disappears in the long haul.