The primary purpose of this study is to statistically characterize the tensile strength of kenaf (Hibiscus cannabinus L.) bast fibers, which are now using for fiber-reinforcing composite materials. This paper is the first part of the present study, and reports the experimental procedures and results for geometrical measurements and monofilament tensile strength test of the kenaf bast fibers sampled from the identical portion in an individual plant. Cross-sectional geometries of the fibers, which in the mesoscopic level consist of elementary fibrous cells and inter-cell materials, were found to vary severely along and among fibers. A method to estimate cross-sectional areas of this plant fiber from their mutually-perpendicular side views was also proposed. Large variations of monofilament strength of the fibers were observed. The strength with gauge length fixed was found to be well described by the two-parameter Weibull distribution in practice, but Weibull shape and scale parameters were different from each of different gauge lengths, which may be primary due to the along-and-among geometrical variation and “mesoscopic” internal structure of the fibers.