Accurate　forward　electrical　characteristics　of　multiquantum-well　(MQW)　lasers　have　been　measured　using　ac　admittance　measurements　together　with　dc　I-V　plot.　The　synchronous　step　offsets　of　apparent　conductance,　apparent　capacitance,　junction　voltage,　series　resistance,　and　ideality　factor　at　the　onset　of　lasing　were　observed　for　the　first　time.,　With　this　effect,　the　lasing　threshold　can　be　deduced　immediately　by　the　LCR　Meter.　It　is　also　found　that　the　junction　voltage　jumps　abruptly　to　a　saturated　value　at　the　onset　of　lasing,　and　the　starting　and　end　points　of　jumping　exactly　correspond　to　the　maximum　of　the　second　derivative　of　lasing　power　with　respect　to　current　and　the　kink　point　of　the　first　derivative,　respectively.　All　of　the　phenomena　may　help　to　verify　and　improve　existing　semiconductor　laser　models.　In　addition,　negative　capacitance　effect　in　LDs　was　observed.