A new method is presented for determining the minimum horizontal principal stress from the pressure decay curve after breakdown during hydraulic fracturing with a constant injection flow rate. The method is developed by the analysis of the growing process of vertically created cracks during hydraulic fracturing being based on the linear fracture mechanics. The method utilizes the plot of the downhole fluid pressure vs the inverse of the cubic root of time since breakdown. The plot fits to a straight line. The line is extrapolated to the time when the time since breakdown is infinite. The pressure value evaluated by the extrapolation gives the magnitude of the minimum horizontal principal stress. In order to verify the present method, laboratory hydraulic fracturing experiments were performed. The results show that the method determines the minimum horizontal principal stress within the error of about 10%.