Employing a bid-ask spread model applicable for order-driven market, this paper decomposes the bid-ask spread of Shanghai Stock Exchange (SSE) into adverse selection and order processing cost components to investigate the relationship between the components of bid-ask spread and order size. It examines the impacts of firm size, price, trading activeness, and volatility on adverse selection cost, and explores the intraday pattern of adverse selection costs and informative trading. Results show that adverse selection costs increase with trade scale. However, order processing costs do not exhibit the economies of scale. Stocks of large firms, which are high-priced and actively traded, have relatively low adverse selection costs; stocks with large volatility have relatively high adverse selection costs. Moreover, this paper finds that the adverse selection component of bid-ask spread in the Chinese stock market exhibits an L-shaped intraday pattern, which implies that heavy trading around market opening is dominated by informative trading, while heavy trading near market closing is dominated by liquidity trading.