diff --git a/nasal_gc.h b/nasal_gc.h
index 8b690d1..45082cd 100644
--- a/nasal_gc.h
+++ b/nasal_gc.h
@@ -304,6 +304,7 @@ struct nasal_gc
     nasal_val*               val_stack[STACK_MAX_DEPTH];
     nasal_val**              stack_top;   // stack top
     std::vector<nasal_val*>  num_addrs;   // reserved address for const vm_num
+    std::vector<nasal_val*>  str_addrs;   // reserved address for const vm_str
     std::vector<nasal_val*>  slice_stack; // slice stack for vec[val,val,val:val]
     std::vector<nasal_val*>  memory;      // gc memory
     std::queue <nasal_val*>  free_list;   // gc free list
@@ -311,7 +312,7 @@ struct nasal_gc
     std::vector<std::unordered_map<int,nasal_val*> > local;
     void                     mark();
     void                     sweep();
-    void                     gc_init(std::vector<double>&);
+    void                     gc_init(std::vector<double>&,std::vector<std::string>&);
     void                     gc_clear();
     nasal_val*               gc_alloc(int);
 };
@@ -386,7 +387,7 @@ void nasal_gc::sweep()
     }
     return;
 }
-void nasal_gc::gc_init(std::vector<double>& nums)
+void nasal_gc::gc_init(std::vector<double>& nums,std::vector<std::string>& strs)
 {
     for(int i=0;i<65536;++i)
     {
@@ -409,13 +410,21 @@ void nasal_gc::gc_init(std::vector<double>& nums)
 
     *val_stack=nil_addr; // the first space will not store any values,but gc checks
 
-    num_addrs.clear(); // init constant numbers
+    // init constant numbers
     for(int i=0;i<nums.size();++i)
     {
         nasal_val* tmp=new nasal_val(vm_num);
         tmp->ptr.num=nums[i];
         num_addrs.push_back(tmp);
     }
+
+    // init constant strings
+    for(int i=0;i<strs.size();++i)
+    {
+        nasal_val* tmp=new nasal_val(vm_str);
+        *tmp->ptr.str=strs[i];
+        str_addrs.push_back(tmp);
+    }
     return;
 }
 void nasal_gc::gc_clear()
@@ -432,6 +441,13 @@ void nasal_gc::gc_clear()
     global.clear();
     local.clear();
     slice_stack.clear();
+
+    for(int i=0;i<num_addrs.size();++i)
+        delete num_addrs[i];
+    num_addrs.clear();
+    for(int i=0;i<str_addrs.size();++i)
+        delete str_addrs[i];
+    str_addrs.clear();
     return;
 }
 nasal_val* nasal_gc::gc_alloc(int type)
diff --git a/nasal_vm.h b/nasal_vm.h
index fd25b1e..2aef50b 100644
--- a/nasal_vm.h
+++ b/nasal_vm.h
@@ -98,7 +98,7 @@ void nasal_vm::init(
     std::vector<double>&      nums,
     std::vector<opcode>&      exec)
 {
-    gc.gc_init(nums);
+    gc.gc_init(nums,strs);
     str_table=strs; // get constant strings & symbols
     exec_code=exec; // get bytecodes
 
@@ -182,9 +182,7 @@ void nasal_vm::opr_pnil()
 }
 void nasal_vm::opr_pstr()
 {
-    nasal_val* val=gc.gc_alloc(vm_str);
-    *val->ptr.str=str_table[exec_code[pc].num];
-    *(++stack_top)=val;
+    *(++stack_top)=gc.str_addrs[exec_code[pc].num];
     return;
 }
 void nasal_vm::opr_newv()
diff --git a/test/bp.nas b/test/bp.nas
index 67304b9..81c6594 100644
--- a/test/bp.nas
+++ b/test/bp.nas
@@ -137,7 +137,7 @@ while(error>0.001)
     }
     cnt+=1;
     show+=1;
-    if(show==300)
+    if(show==250)
     {
         show=0;
         print('epoch ',cnt,':',error,'\r');