Two extended-cavity diode lasers at 780 nm which are longtime frequency-stabilized to Rb87 saturated absorption signals are reported. A high-performance frequency-locking circuit module using a first-harmonic detection technique is designed and achieved. Two lasers are continuously frequency-stabilized for over 100 h in conventional laboratory condition. The Allan standard deviation of either laser is estimated to be 1.3×10^-11 at an integration time of 25 s. The system environment temperature drift is demonstrated to be the main factor affecting long-term stability of the stabilized lasers based on our correlation study between beat frequency and system environment temperature.