Under fairly basic rationales, this paper provides a more general microstructure model of price quotation in an order driven market. Specifically, as an extension of Handa and Schwartz (1996), we decompose the equilibrium of the bid-ask spread, which is derived as a function of the weighted average of three factors including the different valuation of traders and the expected loss of adverse selection respectively from buyers and sellers, into the implicate components which evolve from the characteristics of traders and market competition. More importantly, we can distinguish the expected loss of adverse selection, which is endogenously formed in our model, between from buyers and from sellers and investigate the key determinants of the expected loss of adverse selection. The numerical tests clearly show the relationships between the spread and all exogenous parameters. Furthermore, the empirical tests using transaction data on all listed shares in TAIEX provide strong support for our model and show that the asset volatility and the probability of informed trading are significantly positively related to the adverse selection costs. Our results indicate that on average the different valuations account for approximately 26.02% of the bid-ask spread, seller’s expected loss of adverse selection account for 36.95% , and buyer’s one account for 37.03%.