The fiber size of HEPA (High efficiency particulate air) filters has been drastically decreased in the pursuit of higher collection performance, i.e., high collection efficiency and low pressure drop. The reduction of fiber size as well as the importance of collection of ultrafine particles has raised various problems in particle collection at extremely low Peclet numbers, Pe. The present work focuses on clarifying the filtration problems at low Pe by studying the collection characteristics of ideal model filters. The main findings of the present work are: (1) the log-penetration equation is not valid at low Pe because single fiber efficiency in the vicinity of the filter surface is higher than the inside; (2) filter with a lower packing density has a higher collection performance for Pe<1; (3) the single fiber efficiency of an inclined fiber can be predicted with the prediction equation for perpendicular fibers by defining Pe based on the airflow velocity component normal to the fiber axis; and (4) the effect of inclined fibers on the particle penetration is not significant so the assumption of perpendicular fiber alignment to the main airflow can be introduced without loss in accuracy.