In this study, small sized micro cone penetrometers with 3mm, 5mm and 7mm outer diameters are designed and manufactured in order to evaluate the dimension and shape of the smear zone of prefabricated vertical drain (PVD). The tip resistance is measured by using strain gauges and fiber optic sensors attached near the micro cone tip. The PVD smear zone is investigated by a series of radial penetration tests in the clay specimen prepared in a large-scale consolidometer. Almost constant value of the cone tip resistance is measured within the undisturbed zone. The outer boundary of the smear zone is identified as the location where the deviation of the measured cone tip resistance from the constant value is observed. In addition, the significant change in cone tip resistance is observed in the smear zone with remarkably consistent patterns. This study shows that the PVD smear zone has the asymmetric shape and is effectively detected by using micro penetrometer.