The effect of product structure and availability of assembly instructions were investigated in an experiment. Two products that differed in the order of assembly were used. Both structures had 64 identical parts. One was a vertical assembly built bottom-up, and the other was a hierarchical assembly, which consisted of several subassemblies. Predetermined Motion Time (PMT) analysis showed that the assembly time for both products was identical. Twenty-four test subjects participated in a 2 × 2 factorial between-subject experiment, in which they assembled either of the two products with or without instructions. Because learning and performance are potentially influenced by subject aptitude, cognitive test scores on visual memory, visualization, and spatial orientation were also investigated as influences on manual assembly performance in this task. The hierarchical product took significantly longer time to assemble compared to the vertical. Actual assembly times were also much higher than those predicted using PMT analysis, suggesting that PMT analysis is insufficient to differentiate between designs that differ in complexity. There were no differences between the instructional conditions and individual differences on test scores.