The main objective of the work presented here was to investigate the development of the crack front and the front of the process zone ahead of the crack in the thickness direction. Fatigue crack growth (FCG) tests were conducted using compact type (CT) specimens for two high density polyethylene (PE-HD) pipe grades and were interrupted in the region of stable crack propagation. For the systematic characterization of the process zone, a microtome cut technique was used. Measurements of the process zone length were characterized using microscopic methods in regions of plane stress and plane strain over a wide range of test conditions. In contrast to similar methods used in research reported in the literature, the novelty of this work was the determination of a very dense array of data points in the through-thickness direction. This allowed a very exact characterization of the process zone. The effects of the following test parameters on the process zone length were systematically investigated: stress intensity factor, R-ratio, and frequency. The effective crack length was calculated based on compliance relationships. This methodology shows validity and the calculated data closely fitted the measured data. It is a powerful tool for the dense determination of crack length values that are characteristic for the crack growth mechanism and all necessary data can be easily determined in one and the same FCG experiment, with no need for correction.