Anisotropic magnetoelastic thin films have a significant modification of the acoustic properties near spin reorientation transition (SRT)1. These high sensitivity and nonlinearity of the magnetic system have great potential applications in ultrasonic diagnosis and imaging, magnetic sensor and microactuation2. In this work, we have prepared FeCoSiB (20 nm)/NiFeCu (10-80 nm) bilayers with induced uniaxial anisotropy on Si substrate by magnetron sputtering, and investigated the magnetization dynamics using a low field ferromagnetic resonance (FMR) technique. As shown in Figure 1, the imaginary permeability spectra gradually split from single to double peaks and then to a single peak, when an opposing magnetic field is applied along the easy axis. Fig. 2 shows the variation of the resonance frequencies with the external magnetic field from -150 to 150 Oe. Although the frequency of the single resonance peak satisfies the linear relation of modified Kittel formula3,4, the splitting of resonance peaks was not captured in previous work to the best of our knowledge. Independent on the NiFeCu thickness, this splitting always occurs at certain magnetic field near the coercivity, thus, may be attributed to spin reorientation transition (SRT)5. In the SRT region, the intensity of the low frequency peak decreases with the increase of the reverse magnetic field, while the intensity of the high frequency peak increases continuously, indicating that the Zeeman interaction compensates the anisotropy. Moreover, the field range of SRT region is related to the thickness of bilayer, the effective magnetization, the exchange coupling constant and the magnetoelastic constants. It is found that the FeCoSiB (20 nm)/NiFeCu (30 nm) bilayer has the maximum SRT magnetic field range, where the NiFeCu layer is expected to be near the SRT thickness. These results provide insights into the design of novel straintronic devices and the understanding of magnetization dynamics of ferromagnetic bilayers.