As a key tool in implementing monetary policy and government financing, government bonds play an essential role in financial markets. By means of Islamic financial innovations in the Islamic capital market, the instrument of Islamic treasury bill is published and tradable in the over-the-counter market. Islamic treasury bills have many risks in terms of execution, including the ease of trading and liquidity in the secondary market. Therefore, this research aims to examine some microstructural elements of government bonds using a vector autoregressive model. In this article, the effect of trading volume on bid-ask spread of orders has been investigated. To survey the impact of these variables, the vector autoregressive (VAR) model has been used on intraday data of 17 symbols of Islamic treasury bills in the over-the-counter market, which had the most trading days from 2021 to September 2023. According to the studies, there is a significant relationship between the trading volume and bid-ask spread only in Islamic treasury bills with long-term maturity. Therefore, the effect of the bid-ask spread of orders in different periods is greater than the trading volume, especially in longer-term Islamic treasury bills. Hence, in this research, by analyzing the impulse response function, if there is a shock on the variables, the effect of the trading volume's shock remains for several periods and affects unremarkably the bid-ask spread of orders in most of the short-term and long-term Islamic treasury bills, while effects of the bid-ask spread shocks during initial periods for long-term and short-term Islamic treasury bills is excellent, but decrease sharply during the following periods. These results help traders pay attention and reduce the risk of trading in the over-the-counter market, specifically the long-term treasury bills.
