In Islamic investment, the most distinctive element is the prohibition of interest or riba. In order to avoid riba in investment, Islamic banking and finance has developed profit-loss-sharing systems which claim to be primarily based on Islamic concept. One of the Islamic concepts that involve the profit-loss-sharing system is musyarakah concept. In musyarakah, two or more parties which are known as capital provider and entrepreneur will share the profit and loss during the investment. This study develops the new musyarakah investment model by considering the rate of return as a total of a deterministic profit rate and a function of white noise. This model can be used in forecasting the investment in the stock market. Based on the results obtained, this model is useful in forecasting the musyarakah investment and return for two parties since the results show the accurate forecast investment. The profit from the musyarakah investment, then is compared with profit from single investment that uses geometric Brownian motion (GBM) model. The result shows that the difference profit between these investments is small. It can be concluded that the new musyarakah investment model can be used in order to forecast the profit for both parties with two difference profit sharing rates.