Cellulose aggregate fibrils (CAFs) with dimensions of 100,000×3000×300 nm from unbleached kraft pulp (KP), oxygen delignified KP (KP ox.del ), and fully bleached kraft pulp (BKP) were liberated by a series of high shear and fractionation operations. The CAFs served as microfibril model material to evaluate their dimensional and hygroexpansive behaviors when submitted to variable relative humidities (RHs). The atomic force microscopy images of CAFs from different sources of kraft fibers were obtained during a RH cycle from 50% RH to 78% RH and then to 21% RH while being kept at 23°C. The resulting images were analyzed to determine dimensional changes in length, its concurrent cross-sectional area, width, and height. The mean value of changes in length and width was in the range of 2.3–3.2% and 1.9–3.3%, respectively. The changes in area and height were in the range of 14.5–18.2% and 12.4–17.3%, respectively. The length of CAFs showed a negative correlation with RH, whereas cross-sectional area, width, and height changes correlated positively with RHs. In the out-of-plane direction, such as area and height, the hygroexpansivity was one order of magnitude larger than the in-plane hygroexpansivity, i.e., in terms of length and width.