This paper uses trends and sequences of different firm performance indices to examine the existence of investors' conservatism and representativeness biases in the Taiwan stock market. We construct two performance trend indices. In the global trend index, we find that there exist conservatism and representativeness biases. However, in the local trend index, evidences incline to show that there is no systematic behavioral bias. In addition, we apply a piecewise regression analysis and find that among financial accounting performance indices, only net income exhibits a representativeness bias.