A continuous process for biodiesel preparation was investigated in a pilot plant, using rapeseed oil and methanol as the feedstock, and KOH as the catalyst. The process was equiped with a tubular static mixing reactor and co-solvent such as tetrahydrofuran(THF) and butanone was added in. The experiment showed that the yield of biodiesel was improved evidently when co-solvent was added in. Compared with no co-solvent the conversion ratio of rapeseed oil to biodiesel increased from 56％ to above 80% at feedstock passed the pipe once and the tiptop conversion ratio increased from 76.5% to 94.1% on the condition that molar ratio of methanol to oil was 6︰1, catalyst was 1% weight of oil, the heating temperature was controlled in 250℃ to 270℃, and total flow was 5 L/min. It’s indicated that conversion ratio of rapeseed oil to biodiesel changed little if the dosage of co-solvent reduced properly. All these reaction could reach 90% conversion ratio at 6min to 9min with co-solvent added in.