We incorporate both labor and capital as production inputs and discuss the effects of labor choice on a firm’s optimal investment decision and output dynamics based on real options framework. In particular, we introduce different levels of labor flexibility and examine how it affects the firm’s investment and employment strategy. We show that upward-only adjustable labor, which can arise from employment protection, discourages investment and makes a negative short-run impact on labor employment but a positive impact in the long-run. We also show that the firm invests earlier and produces more at the investment timing when it is either highly labor-intensive or highly capital-intensive. The impact of labor share on output is more pronounced as time elapses and it is more significant when labor is only upward-adjustable than the case of fully flexible labor. Furthermore, we show that uncertainty can make both positive and negative impacts on investment, but that the latter is more pronounced when the labor share is high.