A　microcomputer-based　ultrasonic　gas　flowmeter　with　transit-time　method　is　presented.　Modules　of　the　flowmeter　are　designed　systematically,　including　the　acoustic　path　arrangement,　ultrasound　emission　and　reception　module,　transit-time　measurement　module,　the　software　and　so　on.　Four　200　kHz　transducers　forming　two　acoustic　paths　are　used　to　send　and　receive　ultrasound　simultaneously.　The　synchronization　of　the　transducers　can　eliminate　the　influence　caused　by　the　inherent　switch　time　in　simple　chord　flowmeter.　The　distribution　of　the　acoustic　paths　on　the　mechanical　apparatus　follows　the　Tailored　integration,　which　could　reduce　the　inherent　error　by　2-3%　compared　with　the　Gaussian　integration　commonly　used　in　the　ultrasonic　flowmeter　now.　This　work　also　develops　timing　modules　to　determine　the　flight　time　of　the　acoustic　signal.　The　timing　mechanism　is　different　from　the　traditional　method.　The　timing　circuit　here　adopts　high　capability　chip　TDC-GP2,　with　the　typical　resolution　of　50　ps.　The　software　of　Labview　is　used　to　receive　data　from　the　circuit　and　calculate　the　gas　flow　value.　Finally,　the　two　paths　flowmeter　has　been　calibrated　and　validated　on　the　test　facilities　for　air　flow　in　Shaanxi　Institute　of　Measurement　&　Testing.　(C)　2013　Elsevier　B.V.　All　rights　reserved.