We demonstrate that an electron wave packet produced during intense field ionization can be used for probing molecular dynamics with an attosecond time resolution. When a H 2 molecule is ionized by an intense infrared laser field, a vibrational and an electronic wave packet are simultaneously produced. Using the vibrational wave packet in H as a molecular clock, we deduce the time structure and magnitude of the re-collision electron wave packet. Delaying the re-collision time by changing the laser wavelength, we measure the D vibrational motion with 200 as and 0.05 resolution. We also show that attosecond measurements can be achieved without attosecond optical pulse because of entanglement between vibrational and electron wave packets. Attosecond science will be developed using both optical and electron pulses.