The effect of three processing parameters on the flexural mechanical behavior of chopped bagasse–polyester composites is evaluated. The parameters evaluated were: the size of the chopped material, the pre-treatment derived from the previous processing of the bagasse material on mills for extraction of sugar and alcohol or liquor, and the molding pressure. The results obtained showed that composites fabricated with bagasse with size under mesh #20 sieve, and pre-processed for sugar and alcohol extraction showed the best mechanical performance. This behavior was associated, respectively, with the increase of the surface area and with an almost complete cleaning of the bagasse surface. The molding pressure affects the mechanical behavior only after a threshold value is attained. Its effect was related to the promotion of a closer contact between bagasse and the resin matrix, and the reduction of entrapped voids. The results obtained enable the selection of the best combination of bagasse origin, size, and molding pressure.