A mild etherification of spruce kraft pulp was performed to introduce 1.3 and 2.5 mmol/g carboxyl groups on cellulose chains. 1.3 mmol/g carboxymethyl fibers (CMF) were dissolved partially in alkaline water to form balloons and collars on the tracheid and their ultra-structure was investigated. Primary wall, expanded S1, swollen S2, wrinkled S3, spiral bands of S1, parallel microfibrils of S2 and their transverse splitting were observed on swollen fibers. It is indicated that balloons, collars and wrinkled S3 were formed due to different cellulose microfibril features in different layers of tracheid cell wall. Microspheres with a size up to about 0.6 µm were observed by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). It is shown that they originated from transverse splitting of S2 microfibrils and contain bundles of well-known cellulose nanocrystals (CNC). After homogenization and sonication of an aqueous dispersion of 2.5 mmol/g CMF, electroacoustic spectroscopy showed the presence of nanorods with a size distribution of 18-208 nm. Similar sizes were observed by TEM.