Nanocellulose harvested from biomass has attractive properties that have promoted research on its practical applications. Herein, we investigated nanocellulose-based porous monoliths with oriented microchannels that can be fabricated via a unidirectional freezing method. In this method, water-dispersed cellulose nanofibers (CNFs) were immersed into a cold source at a controlled speed, followed by subsequent freeze-drying. The structure of porous cellulose monoliths mainly depends on two factors: the freezing conditions and properties of the dispersed CNFs. The former has been investigated previously. However, the effects of the latter remain unclear. In this study, CNF suspensions prepared by 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidation cellulose nanofibers (TOCNs) with different aspect ratios and concentrations were used. The effects of these variables on the resulting structure, including the pore shape, size, and wall thickness, were examined. Based on the results, the impact of TOCNs on the structure of porous cellulose monoliths was investigated. Our findings suggested that depending on their structure, the porous cellulose monoliths exhibit different mechanical strengths and mass transport properties. In particular, porous cellulose monoliths synthesized from 5.1 wt.% short TOCNs exhibited a low density (55.9 mg∙cm⁻³), high mechanical strength (8687 kPa), and fast mass transport.