Interfacial performances of fibers/thermoplastic composites are crucial to their mechanical behaviors and application. In this work, isotactic polypropylene (iPP)/various modified glass fibers (GFs) micro composites were prepared under quiescent field and shear field respectively for investigating the interfacial crystallization and mechanical properties. The polyethyleneimine (PEI) modified carbon nanotubes (PEI-CNT) and graphene oxide (PEI-GO) were coated on the GFs surface to form a homogeneous and dense coating layer. Under quiescent field, transcrystallization (TC) structure was easily formed at the interface of iPP/PEI-CNT-GF and iPP/PEI-GO-GF due to the high nucleating ability of PEI-CNT and PEI-GO. The total crystallinity (Xc) increased by 33.0% and 26.8% in comparison to iPP/raw GF. The interfacial shear strength (IFSS) and SEM morphologies of interface after micro-droplet test were adopted to evaluate the interfacial adhesion. The IFSS results improved by 39.5% and 34.5% respectively, which indicated that the sufficient TC structure was beneficial to enhancing interfacial adhesion. In practical production, the preparation process of fibers/thermoplastic composites is under shear condition. It is worthy to study the transcrystallization under shear filed and we researched it further. After pulling fiber at the rate of 50 μm/s, the denser TC structure was formed, and the β-form crystals could be observed, particularly in iPP/PEI-CNT-GF and iPP/PEI-GO-GF micro composites, which was verified by DSC heating curves and selective melting experiment at 158 °C. The dense β-form crystals formation led to higher Xc (51.6% and 49.1%, respectively) and IFSS values (17.9 MPa and 17.5 MPa, respectively) in iPP/PEI-CNT-GF and iPP/PEI-GO-GF. In this work, an effective method was provided to introduce a coating layer of high nucleating capacity on the GFs surface and further construct the different TC structure at the interface of GFs/iPP composites under quiescent field and shear field respectively, which improved the interfacial performance dramatically. It would provide guidance for practical industrial production effectively.