Cellular infiltration into electrospun nanofibers (NFs) is limited due to the dense structure and small pore sizes. We developed a programmed NF collector that can fabricate porous NFs with desired pore sizes and thickness. Previously we demonstrated improved cellular proliferation and differentiation of osteoblasts, osteoclasts, and fibroblasts with increased pore sizes of polycaprolactone (PCL) NF in-vitro. This study investigated in-vivo host cell migration and vascular ingrowth within porous NF sheets implanted subcutaneously in a mouse model. Two types of PCL NFs with well-defined pore sizes were created using varying speeds of the NF collector:​ NF-zero (no movement, pore size 14.4 ± 8.9 µm²) and NF-high (0.232 mm/min, pore size 286.7 ± 381.9 µm²). The NF obtained by using classical flat NF collector (2D NF, pore size 1.09 ± 1.7 µm²) was a control. The three formulae of NFs were implanted subcutaneously in 18 BALB/cJ mice. Animals were killed 7 and 28-days after implantation (n = 3 per group per time point). The tissue with implanted NFs were collected for histologic analysis. Overall, 7-day samples showed little inflammatory response. At 28-days, the degree of tissue penetration of PCL NF sheet matrices was linked to pore size and area. NFs with the largest pore area had more efficient tissue migration and new blood vessel formation compared to those with smaller pore sizes. No newly formed blood vessels were observed in the 2D NF group. A porous NF scaffold with controllable pore size has potential for tissue repair/regeneration in situ with potential for many applications in orthopaedics.