When horizontal wells are drilled underbalanced in coal seam, it was found that borehole which kept stable at the beginning would probably collapse in some time. The phenomenon was called as time delay effect. In order to clarify the concept of time delay effect, the influence of pore pressure on stresses of cleats was analyzed based on the characteristic that cleats are abundant in coal seam, indicating that pore pressure influences the normal stresses of cleat surface while it has no impact on the tangential stresses. Using the data from field production in a CBM well, the influence of pore pressure changes on stability of coal rock was analyzed, indicating that contents of coal fines in drained water increases as pore pressure decreases. Pore pressure changes due to seepage in underbalanced drilling or overbalanced drilling were analyzed. This study above demonstrates that the time delay effect exists absolutely in borehole stability in coal seams during underbalanced drilling. The pore pressure changes are the main factor on the time delay effect, while cleats are the internal cause. In order to analyze further time delay effect quantitatively, the fluid-solid coupling during underbalanced drilling was numerically simulated, and the relationship between pore pressure and time was fitted, and the equation for computing the delaying time was posed by means of borehole stability theory in fracture mechanics. The analysis of the time delay effect can guide the field drilling operation and avoid the instability caused by pore pressure changes. The delaying time can be altered through adjusting bottom hole pressure in order to supply sufficient time for drilling operation. (C) 2010 Elsevier B.V. All rights reserved.